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>

client/ui/loading_screen.gd

@@ -38,6 +38,9 @@ func _build_ui() -> void:
 	_label.mouse_filter = Control.MOUSE_FILTER_IGNORE
 	add_child(_label)
 
+	# client_ver and proto_ver are independent — project.yaml version is the client release,
+	# Protocol.PROTOCOL_VERSION is the wire protocol. Mismatches between builds are visible
+	# only to the observer reading the loading-screen label; a future ticket will surface them.
 	var client_ver := _read_client_version()
 	var proto_ver: int = Protocol.PROTOCOL_VERSION
 	_version_label = Label.new()