Add session shell and close world presentation follow-ups

Session shell
- Startup venue menu (src/game/ui/start_menu.gd): keyboard + mouse, replaces
  the live-shift boot and V as the player-facing venue path (V stays a debug bind).
- Pause overlay on Esc (src/game/ui/pause_overlay.gd): resume, restart shift,
  settings, quit to menu.
- Settings panel (src/game/ui/settings_panel.gd): mouse look sensitivity and
  HUD font scale, persisted via Save into user://settings.json and loaded on boot.
- Shift review "Run another shift" now returns to the venue menu.
- New smoke: src/test/smoke_session_shell.gd (menu -> shift -> pause -> settings
  -> menu), passes headless and windowed.

World and presentation
- Skyline backdrop restored with a transparent asset and an alpha-scissor
  unshaded material; fixes the opaque black slab from the earlier attempt and
  the unimported texture path.
- Steps and landing are a StaticBody3D with per-tread colliders, so the guard
  and NPCs walk up instead of clipping through.
- Marquee Label3D scaled up (72 -> 160) so STEP LINE reads from the sidewalk.
- Focus torus and name tag enlarged for 2-3 m readability.
- Routine traffic despawns after an idle lifetime (NpcAvatar.ephemeral), so the
  16-NPC cap no longer fills with standers; authored events stay for the shift.

Docs and tests
- PLAN.md test/CI section corrected to the custom headless runner and Gitea Actions.
- README controls table, smoke checklist, test count, and status lists refreshed.
- TODO/DONE/MEMORY updated; completed sections moved to DONE.md.
- Headless suite 77 -> 88 passing (settings round-trip, step colliders, skyline
  alpha, session-shell UI behavior, traffic lifetime).

src/test/smoke_session_shell.gd

@@ -0,0 +1,63 @@
+extends SceneTree
+## Session-shell smoke: boot the main scene, confirm the venue menu is showing,
+## start a Joke Shop shift, open the pause overlay and settings, then return to
+## the menu. Exits non-zero on failure.
+##
+## Usage:
+##   godot --headless --path . --script res://src/test/smoke_session_shell.gd
+##   godot --path . --script res://src/test/smoke_session_shell.gd   (windowed)
+var _frames := 0
+var _manager = null
+var _main = null
+
+func _initialize() -> void:
+	var packed: PackedScene = load("res://scenes/main.tscn")
+	if packed == null:
+		printerr("cannot load main scene")
+		quit(1)
+		return
+	_main = packed.instantiate()
+	root.add_child(_main)
+	print("boot: main scene added")
+
+func _process(_delta: float) -> bool:
+	_frames += 1
+	if _frames == 10:
+		_manager = _main.get_node_or_null("GameManager")
+		if _manager.start_menu == null or not _manager.start_menu.visible:
+			printerr("start menu not visible at boot")
+			quit(1)
+			return true
+		print("boot: start menu visible")
+		_manager._on_venue_selected("joke_shop")
+	if _frames == 200:
+		if _manager.npcs.size() == 0:
+			printerr("no NPCs spawned after shift start")
+			quit(1)
+			return true
+		print("boot: shift running, npcs=%d" % _manager.npcs.size())
+		_manager._open_pause()
+	if _frames == 210:
+		if _manager.pause_overlay == null or not _manager.pause_overlay.visible:
+			printerr("pause overlay did not open")
+			quit(1)
+			return true
+		print("boot: pause overlay visible")
+		_manager._open_settings()
+	if _frames == 220:
+		if _manager.settings_panel == null or not _manager.settings_panel.visible:
+			printerr("settings panel did not open")
+			quit(1)
+			return true
+		print("boot: settings visible")
+		_manager.settings_panel._on_cancel()
+		_manager._quit_to_menu()
+	if _frames == 230:
+		if not _manager.start_menu.visible:
+			printerr("quit-to-menu did not return to the start menu")
+			quit(1)
+			return true
+		print("SMOKE PASS: menu -> shift -> pause -> settings -> menu")
+		quit(0)
+		return true
+	return false