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-gui/src/app/pages/browser.rs

@@ -832,8 +832,8 @@ fn show_location_column(ui: &Rc<Ui>, visible: bool) {
     }
 }
 
-/// The menu for one entry, in the order of `docs/UI_UX_PLAN.md` §6: open,
-/// offline, sharing, organising, and the destructive item last on its own.
+/// The menu for one entry, in a fixed order: open, offline, sharing,
+/// organising, and the destructive item last on its own.
 pub(crate) fn entry_context_menu(ui: &Rc<Ui>, entry: &DirEntry) -> ActionMenu {
     let mut menu = ActionMenu::new();
     let (ui_c, entry_c) = (ui.clone(), entry.clone());