docs: rework contributor docs for the 3.0.0 local-first mount

ARCHITECTURE.md described the 2.x write path, where namespace changes
waited for Drive and every mount had its own inode space. Rewrite it
around the local tree (lid, inode = lid + 1, local~<lid> stand-ins), the
queue and its 16 drain workers, sync issues, and the remaining split
between the mount and the mirror engine. Drop the pins.json claim; pins
moved into the database at schema v5.

TESTING.md gains the simulation runs and loses the per-case narrative
that --list and BUGS.md already carry. ROADMAP.md takes over the
post-3.0.0 items from the milestone status file.

Remove MILESTONE-3.0.0-STATUS.md (a working log) and UI_UX_PLAN.md (the
2.x design audit). Code comments and CI that cited the untracked
docs/MILESTONE-3.0.0.md now point at TESTING.md, ARCHITECTURE.md and
ROADMAP.md. Threat-model links follow the section to §10.

crates/pdfs-core/src/config.rs

@@ -157,7 +157,7 @@ pub struct AppConfig {
     /// Record a new folder, a new file, a rename or a delete through the mount
     /// locally and send it from the queue, instead of waiting for Drive in the
     /// syscall. `None` means on. `false` brings back the old path for one
-    /// release (docs/MILESTONE-3.0.0.md §10). Defaulted for configs predating
+    /// release (docs/ROADMAP.md). Defaulted for configs predating
     /// the field.
     #[serde(default)]
     pub local_first: Option<bool>,

crates/pdfs-core/src/db/migrations.rs

@@ -1168,8 +1168,8 @@ const MIGRATION_V35: &str = "
 ALTER TABLE trash ADD COLUMN trashed_at INTEGER;
 ";
 
-/// Schema v36: every node row has a local id (`lid`), phase 2 of
-/// docs/MILESTONE-3.0.0.md.
+/// Schema v36: every node row has a local id (`lid`); see
+/// docs/ARCHITECTURE.md §2.
 ///
 /// The `uid` was the row's key, so a node had no identity until Drive minted
 /// one, and a create that landed moved its row to a new key. The `lid` is given