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.

docs/BUGS.md

@@ -255,7 +255,7 @@ B143 does, after a restart too. Sim test
 
 **Status:** Fixed.
 **Found:** 2026-10-05, checking that every name collision and conflict the drain resolves is
-reported (`docs/MILESTONE-3.0.0.md` §6.2). Each one is in the activity log, and the app words
+reported (sync issues, `docs/ARCHITECTURE.md` §5). Each one is in the activity log, and the app words
 each in the user's language but one.
 
 **Where:** `detail_label` in `crates/pdfs-gui/src/app/pages/activity_text.rs`; the daemon