docs: correct stale multiplayer/C#-backend claims, add TECH_STACK doc

CLAUDE.md and README.md described the pre-multiplayer state (local-only
MVP, planned C# backend) even though server-authoritative multiplayer,
the dedicated server, Docker/CI verification, and Steam transport have
since shipped (Phases 1-6). Update both to reflect reality and add a
docs index in CLAUDE.md pointing at multiplayer-next.md as the current
checklist.

- Add docs/TECH_STACK.md, linked from README, explaining the stack and
  why it's a single GDScript project with no separate backend.
- Add one TODO item for the video settings menu (missing presets/vsync/
  resolution scaling), blocked on the same profiling gate as the
  multiplayer 0.17 tasks.
- Pick up editor-generated .gd.uid sidecars and minor project.godot
  formatting noise from opening the project in Godot 4.7.

TODO.md

@@ -16,6 +16,7 @@ The largest gap between this and a AAA-feeling product is presentation, not code
 
 - [ ] **Audio — there is none.** Zero sound files, zero `AudioStreamPlayer` nodes, no bus layout. Needs: engine hum pitched to throttle, turbo whoosh, ball impacts scaled by collision impulse, wall scrapes, goal explosion, crowd bed, UI clicks, countdown beeps, music. Can be driven off `Ship`'s existing telemetry signals.
 - [ ] Custom font + a real `Theme` resource for the HUD. `ThemeDB.fallback_font` at 10-13 px reads as a debug overlay.
+- [ ] **Video settings menu is missing graphics presets, vsync, and resolution scaling.** `video_settings.gd` currently exposes only AA, glow, and brightness, while SDFGI, SSIL, SSAO, and five shadow-casting lights are on by default and unreachable by the player. Blocked on the same profiling gate as the multiplayer section's 0.16–0.28 tasks below (0.17/0.17b/0.17c/0.17d) — needs a human at the editor with real hardware, not further code changes on its own.
 
 ## Multiplayer (long term)