fix(ui): PR #131 review round 1 — retire starchart, harden registry

Addresses blocker comments from PR review:

- Delete star_map.gd and star_map.tscn — dead implant/map/starchart
  HudGroups registration that should have landed with the atlas
  unification (D-191 criterion 1)
- Remove test_star_map_is_accessible_from_insert_ui and
  test_star_map_scene_exists from test_sprint30.gd — D-191 supersedes
  the insert-UI accessibility pattern
- ImplantRegistry._scan() now detects default_key collisions
  (first-wins with push_warning) and validates default_mode against
  HudGroups.Mode enum (skip + warn on invalid)

star_map_data.json remains — still used by atlas_app and
economics_app for system index lookups.

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

client/ui/implant/implant_registry.gd

@@ -17,6 +17,8 @@ func get_manifests() -> Array:
 func _scan() -> void:
 	_scanned = true
 	_manifests.clear()
+	var key_owners: Dictionary = {}  # default_key -> app_path (collision detection)
+	var valid_modes: Array = HudGroups.Mode.values()
 	var dir := DirAccess.open("res://ui/implant/apps")
 	if dir == null:
 		push_warning("ImplantRegistry: could not open res://ui/implant/apps")
@@ -28,10 +30,28 @@ func _scan() -> void:
 			var tres_path: String = "res://ui/implant/apps/%s/app.tres" % entry
 			if ResourceLoader.exists(tres_path):
 				var m = load(tres_path)
-				if _is_valid_manifest(m):
-					_manifests.append(m)
-				else:
+				if not _is_valid_manifest(m):
 					push_warning("ImplantRegistry: invalid or missing app_path in %s" % tres_path)
+				else:
+					var app_path: String = m.get("app_path")
+					var default_mode: int = m.get("default_mode", 2)
+					var default_key: int = m.get("default_key", -1)
+					if not valid_modes.has(default_mode):
+						push_warning(
+							"ImplantRegistry: invalid default_mode %d in %s — skipping" % [
+								default_mode, tres_path
+							]
+						)
+					elif default_key >= 0 and key_owners.has(default_key):
+						push_warning(
+							"ImplantRegistry: key binding collision — key %d already bound to %s, skipping %s" % [
+								default_key, key_owners[default_key], app_path
+							]
+						)
+					else:
+						if default_key >= 0:
+							key_owners[default_key] = app_path
+						_manifests.append(m)
 		entry = dir.get_next()
 	dir.list_dir_end()