chore(multiplayer): Phase 0 refactors + graphics/perf settings groundwork

Lands the non-networked Phase 0 tasks from multiplayer-todo.md (ship/camera/
arena refactors, sim constants, background FPS handling) plus a first pass
at exposing graphics/performance settings (presets, resolution scaling,
vsync, FPS cap, perf overlay) and a GPU profiling harness for the
real-hardware follow-up in task 0.15b.

TODO.md

@@ -19,7 +19,10 @@ The largest gap between this and a AAA-feeling product is presentation, not code
 
 ## Multiplayer (long term)
 
-- [ ] `RemoteShipController extends ShipController` — feeds replicated `ShipAction`s from a network peer into the local ship simulation.
-- [ ] Networked `GameMode` subclass: per-peer ship spawning (MultiplayerSpawner or custom), authoritative server for ball/score.
-- [ ] C# backend / online servers per README roadmap (not started).
-- [ ] Possible v0.2 split-screen: spawn one `ship_camera_rig` + viewport per local player (camera is already outside the ship scene to allow this).
+Planned in **[`multiplayer-todo.md`](multiplayer-todo.md)** — architecture decisions (server-authoritative dedicated servers, client-side prediction, ENet then Steam), wire format, latency budget, and an eight-phase task breakdown. Nothing implemented yet.
+
+Phase 0 of that plan is a set of non-networked refactors that land independently and are verifiable in single-player today; start there. It now also carries the **graphics/performance work** — the project has never been profiled, and `video_settings.gd` exposes only AA, glow and brightness while SDFGI, SSIL, SSAO and five shadow-casting lights are on by default and unreachable (see §5.5 there).
+
+**Tasks 0.1–0.15, 0.18–0.25, 0.27, 0.29 are done** (see the Phase 0 table in `multiplayer-todo.md` for what each one actually changed — several deviated from the original plan for concrete GDScript/Godot reasons recorded inline). Remaining, all blocked on **0.15b (profile, on reference hardware, in the live editor — not done)**: 0.16 (camera to `_process`), 0.17/0.17b/0.17c/0.17d (graphics presets, vsync, resolution scaling), **0.26 (bake the arena GI to retire SDFGI — the largest frame-time win available, costs no image quality since the arena is fully static)**, and 0.28 (physics separate-thread prototype, flagged as the riskiest task in the phase). These need a human at the editor with real hardware to profile and eyeball, not further code changes.
+
+- [ ] Possible v0.2 split-screen: spawn one `ship_camera_rig` + viewport per local player (camera is already outside the ship scene to allow this). Unrelated to online play.