feat(hud): F3 debug overlay — FPS, frame time, worst frame, state

A top-left readout for performance work: smoothed FPS and frame time, the worst
frame in a rolling two-second window, the active view, player speed, a frame counter
and the player's position in metres from the project origin.

- **F3** toggles it, through the console variable  so a packaged build
  can be checked without a rebuild ().
- **On automatically** for  and , so the performance capture never
  depends on pressing a key first; **off in the built demo**, because a player should
  not see an FPS counter over Hollywood Boulevard.
- The first two seconds are a **warm-up window** excluded from the worst-frame figure.
  Without it the readout showed 'worst 400 ms' for a healthy run: the startup shader
  and streaming hitches dominated the rolling maximum.

Measured cost on the self-test route at 1440p: 2.64 -> 2.77 ms average, about 5 %,
still 6x the floor. The release-check number stays the overlay-off run; the overlay-on
series is committed beside it as Docs/perf/0.2.0-walk-frametimes-overlay.csv.

Docs: controls (F3 row, overlay section, switch table), release checks (cost row),
architecture building block view (HUD and input list).

Docs/release-checks.md

@@ -43,6 +43,7 @@ Measured at 1440p on the development workstation: Bazzite, Ryzen 7 7700X, 64 GB
 | `0.1.0` | Collision and packaging | PASS — no blocking collision, no invisible walls, no seam gaps (see audit note) | `-qaselftest` on the packaged build, 2026-11-08 | 2026-11-08 |
 | `0.1.0` | 60 FPS at 1440p | PASS — 2.42 ms avg (414 fps), p99 3.01 ms at 2560×1440, TSR on, VSync off, no FG | `-qaselftest` perf capture, `Docs/perf/0.1.0-walk-frametimes.csv` | 2026-11-08 |
 | `0.2.0` | 60 FPS at 1440p | PASS — 2.64 ms avg (379 fps), p50 2.54, p95 3.39, p99 3.76, max 4.05 ms at 2560×1440, TSR on, VSync off, no FG | `-qaselftest` perf capture, `Docs/perf/0.2.0-walk-frametimes.csv` | 2026-11-09 |
+| `0.2.0` | Debug overlay cost | PASS — same route with the F3 overlay drawn: 2.77 ms avg (361 fps), p50 2.64, p95 3.52, p99 3.87, max 4.43 ms; about **5 %** heavier, still 6× the floor. The overlay is on for `-perf`/`-qaselftest`, off in the built demo | `Docs/perf/0.2.0-walk-frametimes-overlay.csv` | 2026-11-09 |
 | `0.1.0` | Fresh checkout build | PASS — clean clone of `main` (`4bc41e9`), `git lfs pull` + `check-lfs.sh` OK (130 LFS files), both `GigLineEditor` and `GigLine` targets built with the documented engine | `/tmp/fresh-check`, 2026-11-08 | 2026-11-08 |
 | `0.1.0` | Packaged Linux build | PASS — packaged Development build launches straight into DemoMap, no missing-asset/linker errors; controls shown in-game and in `Docs/controls.md` | measurement recorded 2026-11-08; the capture was removed from the repository 2026-11-09 | 2026-11-08 |