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/scripts/main.gd

@@ -178,14 +178,19 @@ func _unhandled_key_input(event: InputEvent) -> void:
 		return
 	var key_event := event as InputEventKey
 	# Registry-driven toggle: each manifest declares its own default_key.
-	for manifest: Variant in ImplantRegistry.get_manifests():
-		if manifest.get("default_key") == key_event.keycode:
+	for manifest: ImplantAppManifest in ImplantRegistry.get_manifests():
+		if manifest.app_path.is_empty():
+			push_warning("main.gd: manifest with empty app_path — skipping")
+			continue
+		if manifest.default_key == key_event.keycode:
 			HudGroups.toggle_app(
-				manifest.get("app_path"),
-				ImplantRegistry.get_resolved_mode(manifest.get("app_path", ""))
+				manifest.app_path,
+				ImplantRegistry.get_resolved_mode(manifest.app_path)
 			)
 			return
-	# [ / ] — cycle economics system selector (app-specific, not generic enough for manifest).
+	# [ / ] — in-app navigation for the economics monitor. Not manifest-declared because
+	# these control intra-app navigation (prev/next system), not app launch. A planned
+	# handle_global_key lifecycle hook will absorb this (see arch doc Follow-up).
 	if key_event.keycode == KEY_BRACKETLEFT:
 		if economics_app and HudGroups.is_app_active("implant/economics"):
 			economics_app.navigate(-1)
@@ -195,8 +200,9 @@ func _unhandled_key_input(event: InputEvent) -> void:
 
 
 func _on_atlas_economics_link(system_id: String) -> void:
-	# #835 D-191: Pre-filter the economics monitor to the city's system and pop
-	# the panel open. AtlasApp closes itself before emitting this signal.
+	# #835 D-191: Pre-filter the economics monitor to the city's system and open
+	# it as an insert panel. AtlasApp closes automatically when Economics opens
+	# (HudGroups single-active-app rule → app_changed signal).
 	if economics_app == null:
 		return
 	economics_app.select_system(system_id)