fix(ui): PR #131 review round 3 — nits sweep

Seven nit-level fixes from PR review:

- Remove dead signal insert_deactivated from ImplantApp; the hook method
  on_insert_deactivated() is the actual contract.
- Rewrite _on_atlas_economics_link comment in main.gd to reflect the
  actual flow (AtlasApp closes as a consequence of HudGroups single-
  active-app, not before emitting anything).
- Document the "pop never empties below default" invariant on
  ImplantNavStack.pop() with a pointer to reset_to_default.
- Add _mutating re-entrancy guard on ImplantNavStack mutation methods.
  push_error + early return if called during a screen_changed emission.
- main.gd registry loop now uses typed ImplantAppManifest property
  access (manifest.app_path, manifest.default_key) instead of
  dictionary-style .get() calls. Empty app_path triggers push_warning.
- Document the economics [/] hotkey exception in main.gd and reference
  the planned handle_global_key lifecycle hook. Arch doc Follow-up
  section gains a bullet for the new hook.
- Comment the independent-version-read rationale above client_ver and
  proto_ver in loading_screen.gd.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/architecture/implant-app-pattern.md

@@ -297,6 +297,7 @@ Autoload order: `implant_registry` must scan before `main.gd` queries manifests
 - **`DataChannels` autoload** — manifest-declared snapshot subscriptions. Seam already reserved via `on_install` lifecycle position.
 - **Launcher UI** — once a fourth implant app exists, a chooser becomes necessary. Until then, key bindings suffice.
 - **Settings-UI key remapping** — resolves key-binding collisions beyond first-wins.
+- **`handle_global_key` lifecycle hook** — new `ImplantApp` override; shells handle intra-app keys (e.g. economics `[`/`]` system navigation). Main.gd routes unhandled keydown to the active app instead of hardcoding per-app bindings.
 
 ### Deferred (Phase 6+, not sprint-scoped)