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).

PLAN.md

@@ -432,13 +432,15 @@ The venue-driven architecture maps onto Godot as follows:
 
 ## Testing
 
-- Unit and scenario tests use the GUT (Godot Unit Test) framework.
+- Unit and scenario tests use a lightweight custom headless runner
+  (`src/test/test_runner.gd`). GUT was evaluated but requires the editor plugin.
 - Tests run in headless mode so rules, access verification, escalation, and scoring can be verified without rendering or user input.
 
 ## Version Control and CI
 
 - Source is tracked with git.
-- GitHub Actions runs headless Godot tests and, in later stages, exports desktop builds.
+- Gitea Actions (`.gitea/workflows/build.yml`) runs headless Godot tests on push/PR and exports
+  Linux/Windows/macOS desktop builds plus the NSIS installer and AppImage on tags.
 
 ## Planned Commands
 
@@ -446,7 +448,7 @@ The following commands apply once the Godot project is scaffolded:
 
 - Open the editor: `godot --editor --path .`
 - Run the game: `godot --path .`
-- Run tests headlessly via GUT.
+- Run tests headlessly: `godot --headless --path . res://src/test/test_runner.tscn`
 
 ## Export Targets