fix: make the template workflow real, and the UI legible

The headline flow — fill a config from its template — did not work. Three
bugs, each individually enough to make the tool feel unfinished:

- `add_missing` was unreachable. `find_binding` returned the first match in
  a linear array and `edit_append` was also bound to `a`, 17 entries earlier.
  Bindings now resolve through one table built from the config, and
  `KeybindsConfig::conflicts()` reports any key bound twice instead of
  letting one silently shadow the other. `add_missing` moves to `<Space>`,
  with `A` to take every missing default at once.
- Item status was ignored on write. Every writer fell back to
  `template_value`, so "(missing)" keys landed in the output regardless and
  saving was close to a copy. Unaccepted entries are now omitted.
- Parse errors were swallowed, then the good file was overwritten. A
  malformed target opened as an empty editor and `:w` truncated it. Parse
  failures are now fatal, reported with context, before the TUI starts.

Also:

- Split "differs from the template" (ItemStatus::Overridden) from "you
  edited this" (ConfigItem::dirty). A real config differs from its example
  on nearly every line, so the old Modified flag painted the whole list
  orange and carried no signal.
- Splice template-only entries in at their natural position instead of
  appending them, so a missing nested key no longer strands itself at the
  bottom of the tree at depth 3.
- Preserve .env comments and blank lines across a save.
- Header naming the output file and the template it is compared against,
  plus missing/changed counts. Previously nothing on screen said which
  files were in play or where `:w` would write.
- `?` opens a two-column keybinding reference built from the user's own
  config, scrollable on short terminals. The status bar drops to a few
  contextual hints; the old one joined 11+ hints into an unwrapped line
  that truncated at 80 columns.
- Foldable groups, which the `+ ` prefix had always implied.
- Search covers values as well as keys, reports `match n/m`, and `Esc`
  restores the pre-search cursor.
- Quitting with unsaved changes is refused; added `:q!`, `:x`, `:w <path>`.
- Status messages expire after 4s instead of permanently replacing hints.
- Cancelling a rename on a just-created item removes it again.
- Editing a Number field with text demotes the type instead of failing
  obscurely at write time.
- Undo-tree branches are reachable via `g-`/`g+` and reported on undo.
- Poll instead of blocking, so resizes repaint; drop the unused mouse
  capture that was costing terminal text selection for nothing.
- `--format` accepts xml, ini and properties, which were already supported.

36 tests, up from 13.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01C5mrbEFmwv2A4cdzbcSiS1

README.md

@@ -5,10 +5,12 @@
 ## Features
 
 - **Universal Format Support**: Read, edit, and write `.env`, `JSON`, `YAML`, `TOML`, `XML`, `INI`, and `Properties` files seamlessly.
-- **Tree View Navigation**: Edit nested data structures in a beautiful, depth-colored tree view.
+- **Tree View Navigation**: Edit nested data structures in a depth-colored tree view, with foldable groups so a large `docker-compose.yml` stays readable.
 - **Smart Template Discovery**: Rule-based resolver automatically discovers relationships (e.g., `.env.example` vs `.env`, `config.template.properties` vs `config.properties`) and highlights missing keys.
 - **Strict Type Preservation**: Intelligently preserves data types (integers, booleans, strings) during edit-save cycles, ensuring your configuration stays valid.
-- **Add Missing Keys**: Instantly pull missing keys and their default values from your template into your active configuration with a single keystroke.
+- **Nothing Written Behind Your Back**: Keys that exist only in the template are marked `(missing)` and stay out of the output until you pull them in with `<Space>` (or `A` for all of them).
+- **Comment Preservation**: `.env` comments and blank lines survive a save, so the documentation in your `.env.example` is not the price of using the tool.
+- **Discoverable**: A header names the file being written and the template it is compared against; `?` opens the full keybinding reference, built from *your* config.
 - **Advanced Undo/Redo Engine**: Features a tree-based undo/redo history that ensures you never lose changes during complex branching edits. It properly tracks all modifications, including nested renames and item additions/deletions.
 - **Vim-inspired Workflow**: Navigate with `j`/`k`, `gg`/`G`, edit with `i`, search with `/`, and save with `:w`.
 - **Modern UI**: A polished, rounded interface featuring a semantic Catppuccin Mocha palette with support for terminal transparency.
@@ -69,36 +71,59 @@ Open any configuration file in Neovim and run `:Mould`. It will launch a floatin
 
 ### Keybindings (Default)
 
-- **Normal Mode**
-  - `j` / `Down`: Move selection down
-  - `k` / `Up`: Move selection up
-  - `gg`: Jump to the top
-  - `G`: Jump to the bottom
-  - `i`: Edit value (cursor at start). If selected is a group, enters rename mode.
-  - `a`: Edit value (cursor at end)
-  - `s` / `S`: Substitute value (clear and edit)
-  - `r`: Rename the current key or group. (Cannot rename array indices).
-  - `o`: Append a new item (as a sibling or array element). Enters rename mode for non-array items.
-  - `O`: Prepend a new item (as a sibling or array element). Enters rename mode for non-array items.
-  - `alt+o`: Append a new group/object as a child. Enters rename mode for the new group.
-  - `alt+O`: Prepend a new group/object as a child. Enters rename mode for the new group.
-  - `t`: Toggle the selected item between a group/object and a standard variable.
-  - `dd`: Delete the currently selected variable or group (removes all nested children).
-  - `u`: Undo the last change
-  - `U`: Redo the reverted change
-  - `a`: Add missing value from template to active config
-  - `/`: Search for configuration keys
-  - `n` / `N`: Jump to next / previous search match
-  - `:w` or `Enter`: Save the current configuration
-  - `:q` or `q`: Quit the application
-  - `:wq`: Save and quit
-  - `Esc`: Clear current command prompt
+Press `?` inside `mould` for the same list, rendered from your own configuration.
 
-- **Insert / Rename Modes**
-  - Type your value/key string.
-  - Arrow keys: Navigate within the input field
-  - `Enter`: Commit the value/key and return to Normal Mode. If renaming, checks for key collisions.
-  - `Esc`: Cancel edits and return to Normal Mode. Reverts changes to the current field.
+**Navigate**
+
+- `j` / `k` (or arrow keys): Move the selection
+- `gg` / `G`: Jump to the top / bottom
+- `Tab` or `za`: Fold or unfold the selected group
+- `zM` / `zR`: Fold / unfold everything
+- `/`: Search over keys *and* values. `Enter` keeps the query for `n` / `N`; `Esc` cancels and restores the cursor
+- `n` / `N`: Jump to the next / previous match
+
+**Edit**
+
+- `i`: Edit the value with the cursor at the start. On a group, renames it instead
+- `a`: Edit the value with the cursor at the end
+- `s` / `S`: Replace the value (clears it first)
+- `r`: Rename the current key or group (array indices cannot be renamed)
+
+**Structure**
+
+- `o` / `O`: Add an item below / above (as a sibling, or as an array element)
+- `alt+o` / `alt+O`: Add a child group below / above
+- `t`: Toggle the selected item between a group and a plain value
+- `dd`: Delete the selected item and everything nested under it
+
+**Template**
+
+- `Space`: Take the template default for the selected `(missing)` entry
+- `A`: Take every missing template default at once
+
+**History**
+
+- `u` / `U`: Undo / redo
+- `g-` / `g+`: Switch between alternative branches of the undo tree
+
+**Session**
+
+- `?`: Toggle the keybinding reference
+- `:`: Open the command prompt
+- `q`: Quit (refused while there are unsaved changes)
+
+**Commands**
+
+- `:w`: Write the output file
+- `:w <path>`: Write somewhere else, and keep writing there
+- `:q`: Quit; `:q!` quits and discards changes
+- `:wq` or `:x`: Write and quit
+
+**Insert / Rename Modes**
+
+- Type your value or key. Arrow keys navigate within the field
+- `Enter`: Commit and return to Normal mode. Renames are checked for collisions
+- `Esc`: Cancel. An item you just created and never named is removed again
 
 ---
 
@@ -131,14 +156,17 @@ tree_depth_3 = "#89b4fa"
 tree_depth_4 = "#fab387"
 
 [keybinds]
+# A binding may list alternatives separated by commas.
+# Named keys are written in angle brackets: <Space>, <Tab>.
 down = "j"
 up = "k"
 edit = "i"
 edit_append = "a"
-edit_substitute = "s"
+edit_substitute = "s,S"
 save = ":w"
 quit = ":q"
-normal_mode = "Esc"
+quit_key = "q"
+command = ":"
 search = "/"
 next_match = "n"
 previous_match = "N"
@@ -149,10 +177,18 @@ prepend_item = "O"
 delete_item = "dd"
 undo = "u"
 redo = "U"
+undo_branch_prev = "g-"
+undo_branch_next = "g+"
 rename = "r"
 append_group = "alt+o"
 prepend_group = "alt+O"
 toggle_group = "t"
+add_missing = "<Space>"
+accept_all = "A"
+toggle_fold = "<Tab>,za"
+fold_all = "zM"
+unfold_all = "zR"
+help = "?"
 ```
 
 ---
@@ -165,7 +201,8 @@ toggle_group = "t"
 
 -   **Separation of Concerns**: Clear boundaries between UI rendering, application state, input handling, and file format logic.
 -   **Unified Data Model**: All parsed configuration data is normalized into a single `Vec<ConfigItem>` structure, simplifying application logic across different file types.
--   **Vim-inspired Modality**: The application operates in distinct modes (`Normal`, `Insert`, `InsertKey`, `Search`), each with specific keybinding behaviors, enabling efficient interaction.
+-   **Vim-inspired Modality**: The application operates in distinct modes (`Normal`, `Insert`, `InsertKey`, `Search`, `Help`), each with specific keybinding behaviors, enabling efficient interaction.
+-   **One Keybinding Table**: Every Normal-mode key resolves through `KeybindsConfig::resolved()`, so a rebind takes effect in the dispatcher, the status-bar hints and the help overlay at once. `KeybindsConfig::conflicts()` reports any key bound to two actions rather than letting one silently shadow the other.
 -   **Non-linear Undo/Redo**: A robust undo tree allows users to revert and re-apply changes across complex branching edit histories.
 
 ### Key Components
@@ -181,8 +218,9 @@ toggle_group = "t"
         *   `path`: A `Vec<PathSegment>` (composed of `PathSegment::Key(String)` for object keys and `PathSegment::Index(usize)` for array indices) that defines its full hierarchical location.
         *   `value`: `Option<String>` holding the actual value.
         *   `is_group`: A boolean indicating if this item is a structural node (object or array).
-        *   `status`: (`ItemStatus::Present`, `MissingFromActive`, `Modified`) reflecting comparison with a template.
+        *   `status`: (`ItemStatus::Present`, `MissingFromActive`, `Overridden`) reflecting comparison with a template. Whether *you* edited an entry is tracked separately by `dirty`, so a config that legitimately differs from its example file does not render as if every line had just been touched.
         *   `value_type`: (`ValueType::String`, `Number`, `Bool`, `Null`) to ensure type preservation during writes.
+        *   `collapsed` / `leading_trivia`: fold state, and the comment lines that preceded the entry in the source file.
 
 3.  **Format Handlers (`src/format/*`)**
     *   Each file format has a dedicated handler (`EnvHandler`, `IniHandler`, `HierarchicalHandler`, `PropertiesHandler`) implementing the `FormatHandler` trait.
@@ -195,7 +233,7 @@ toggle_group = "t"
 
 5.  **Terminal UI & Event Loop (`src/ui.rs` & `src/runner.rs`)**
     *   **UI Rendering (`src/ui.rs`)**: Powered by the `ratatui` library. Renders a flexible layout consisting of a styled hierarchical list, an active input field for editing, and a dynamic status bar.
-    *   **Event Runner (`src/runner.rs`)**: Powered by `crossterm`. It intercepts raw keyboard events, resolves multi-key sequences (like `dd` or `gg`), delegates character input to the `tui-input` backend during active editing, and dispatches actions to mutate the `App` state. It includes logic to prevent "one-key-behind" issues and manage complex keybindings like `alt+o`.
+    *   **Event Runner (`src/runner.rs`)**: Powered by `crossterm`. It polls for events rather than blocking, so terminal resizes and expiring status messages repaint without waiting for a keystroke. It resolves multi-key sequences (like `dd` or `gg`), delegates character input to the `tui-input` backend during active editing, and dispatches actions to mutate the `App` state.
 
 6.  **Neovim Plugin (`lua/mould/init.lua`)**
     *   Implements a Lua wrapper that calculates terminal geometries and launches the CLI `mould` binary inside an ephemeral, floating terminal buffer, ensuring automatic Neovim `checktime` synchronization upon `mould`'s exit.