Remove the dead Slay-the-Spire map system (superseded by the lobby)

The lobby hub replaced the between-fights map, leaving the whole map
subsystem unused. Remove it:

- Delete MapScreen.tscn + map_screen.gd (the dead screen) and map_node.gd
  (the MapNode class).
- Strip map generation + navigation from run_state.gd (~90 lines:
  _generate_map/_link_rows/reachable/select_node/node_at/current_node/
  run_complete/complete_current_level, plus map/player_row/col/pending_row/col).
  ensure_run() now guards on a _run_active flag instead of the map array, so
  the shell reloading for each fight still can't wipe the run in progress.
- Simplify active_level() to the lobby flow (override -> lobby -> gate -> arena).
- Drop the now-unread LevelDef.color / is_boss fields and the _make_level args.
- Remove the map test from logic_test.gd.

Live flow unchanged (lobby gates via choose_level, wins via return_to_lobby).
Full headless suite green; menu + shell boot into the lobby clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FBbGXbzg3SD6ZBSadPPF5H

levels/level_def.gd

@@ -1,9 +1,8 @@
 class_name LevelDef
 extends Resource
-## Definition of one playable level on the run map — the arena today, later a cave with
-## a bear, a china shop to smash, and so on. Every map node references a LevelDef, and
-## the map/flow code never hard-codes a scene: adding a level is a new LevelDef (plus its
-## scene) handed out by the map generator in run_state.gd.
+## Definition of one playable level — the arena, the bear's den, the lobby hub, and later
+## a china shop to smash. The flow code never hard-codes a scene: a level is a LevelDef
+## (plus its scene) built in run_state.gd and served through the lobby.
 
 @export var id: StringName = &""
 @export var display_name: String = "Level"
@@ -15,14 +14,10 @@ extends Resource
 ## colliders and any level-specific subscripts. Swapped wholesale between levels so no
 ## previous level's geometry (or its colliders) can leak into the next.
 @export var level_scene: PackedScene = null
-## Placeholder pip tint on the map until per-level icon art exists.
-@export var color: Color = Color(0.80, 0.62, 0.24)
 ## Camera framing multiplier when this level loads — 1.0 is the shell camera's default
 ## shot; >1 dollies the camera back for a wider view (the lobby pulls out so the whole
 ## hub reads). Applied by game_shell; independent of the player's scroll-wheel zoom.
 @export var camera_zoom: float = 1.0
-## A boss level caps a run — its node is drawn larger and clearing it ends the map.
-@export var is_boss: bool = false
 ## The enemy this level pits the bull against. Null → the usual matador wave (the
 ## spawner's default). Set → a single boss enemy of this scene is spawned instead (e.g.
 ## the bear), and the fight is won when it falls. The arena scene is shared either way;