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/implant/implant_app.gd

@@ -11,7 +11,6 @@ extends Control
 
 signal app_opened(mode: int)
 signal app_closed
-signal insert_deactivated
 
 var manifest: ImplantAppManifest = null
 var nav: ImplantNavStack = null

client/ui/implant/implant_nav_stack.gd

@@ -8,6 +8,7 @@ signal screen_changed(current_screen_id: String)
 var _stack: Array[String] = []
 var _payloads: Array[Dictionary] = []
 var _default_screen_id: String = ""
+var _mutating: bool = false  # re-entrancy guard: set while screen_changed is emitting
 
 
 func set_default(screen_id: String) -> void:
@@ -15,30 +16,53 @@ func set_default(screen_id: String) -> void:
 
 
 func push(screen_id: String, payload: Dictionary = {}) -> void:
+	if _mutating:
+		push_error(
+			"ImplantNavStack: nested mutation detected — use call_deferred from screen_changed handler"
+		)
+		return
 	_stack.append(screen_id)
 	_payloads.append(payload)
+	_mutating = true
 	screen_changed.emit(screen_id)
+	_mutating = false
 
 
+# pop() never empties the stack below the default screen — this app always
+# has at least one screen visible. To truly reset, use reset_to_default().
 func pop() -> void:
+	if _mutating:
+		push_error(
+			"ImplantNavStack: nested mutation detected — use call_deferred from screen_changed handler"
+		)
+		return
 	if _stack.is_empty():
 		push_warning("ImplantNavStack: pop() on empty stack")
 		return
 	_stack.pop_back()
 	_payloads.pop_back()
 	if not _stack.is_empty():
+		_mutating = true
 		screen_changed.emit(_stack.back())
+		_mutating = false
 	elif not _default_screen_id.is_empty():
 		push(_default_screen_id)
 
 
 func replace(screen_id: String, payload: Dictionary = {}) -> void:
+	if _mutating:
+		push_error(
+			"ImplantNavStack: nested mutation detected — use call_deferred from screen_changed handler"
+		)
+		return
 	if not _stack.is_empty():
 		_stack.pop_back()
 		_payloads.pop_back()
 	_stack.append(screen_id)
 	_payloads.append(payload)
+	_mutating = true
 	screen_changed.emit(screen_id)
+	_mutating = false
 
 
 func reset_to_default() -> void:

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()