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.

Game/tests/cases/test_arena_rotation.gd.uid

@@ -0,0 +1 @@
+uid://cklv8t2htg2gf

Game/tests/cases/test_net_transport.gd.uid

@@ -0,0 +1 @@
+uid://dtqbio4ob00hq

Game/tests/cases/test_project_settings.gd.uid

@@ -0,0 +1 @@
+uid://dvwivtlgx2f34

Game/tests/export_server_smoke.gd.uid

@@ -0,0 +1 @@
+uid://bf7o5x2nug4br

Game/tests/steam_template_smoke.gd.uid

@@ -0,0 +1 @@
+uid://by4f1ydqsm0ry