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/autoload/save.gd

@@ -37,3 +37,29 @@ func save_shift_result(result: Dictionary) -> Error:
 	var results := load_shift_results()
 	results.append(result)
 	return _save_json(SHIFT_RESULTS_FILE, results)
+
+## Defaults for player settings. Kept in code so a missing or partial
+## settings.json still produces a usable config.
+const DEFAULT_SETTINGS := {
+	"look_sensitivity": 0.0022,
+	"hud_font_scale": 1.0,
+	"ai_difficulty": "normal",
+}
+
+## Load player settings, merged over the defaults. Returns a Dictionary.
+func load_settings() -> Dictionary:
+	var merged: Dictionary = DEFAULT_SETTINGS.duplicate(true)
+	var data = _load_json(SETTINGS_FILE)
+	if data is Dictionary:
+		for key in data.keys():
+			if merged.has(key):
+				merged[key] = data[key]
+	return merged
+
+## Persist player settings. Returns Error.
+func save_settings(settings: Dictionary) -> Error:
+	var merged: Dictionary = DEFAULT_SETTINGS.duplicate(true)
+	for key in settings.keys():
+		if merged.has(key):
+			merged[key] = settings[key]
+	return _save_json(SETTINGS_FILE, merged)