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.

Game/scripts/training_mode.gd

@@ -1,5 +1,6 @@
 class_name TrainingMode
 extends GameMode
+const SimConstants = preload("res://scripts/sim_constants.gd")
 
 # Headless self-play training mode: two RL-driven ships, no HUD, no camera.
 # The scene also contains the godot_rl_agents Sync node, which speaks TCP to
@@ -120,8 +121,9 @@ const MAX_RANDOM_SHIP_SPEED := 8.0
 # marker spacing, which uses the same margin for the same reason.
 const MIN_SHIP_SEPARATION := 4.5
 
-# Sim runs at 60 physics ticks per sim-second regardless of speedup.
-const TICKS_PER_SIM_SECOND := 60.0
+# Sim runs at SimConstants.TICK_HZ physics ticks per sim-second regardless
+# of speedup.
+const TICKS_PER_SIM_SECOND := float(SimConstants.TICK_HZ)
 
 var _agents: Array[ShipAIController] = []