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/ship_action.gd

@@ -9,3 +9,15 @@ extends RefCounted
 var thrust := Vector3.ZERO    # Per-axis -1..1: x = strafe, y = vertical, z = forward/back
 var rotation := Vector3.ZERO  # Per-axis -1..1: x = pitch, y = yaw, z = roll
 var turbo := false
+
+
+# Returns a distinct ShipAction with equal fields. Callers that hold onto an
+# action past the tick it was returned in (input history, prediction ring)
+# must copy() it — get_action() implementations are free to return a reused
+# instance, and player_ship_controller.gd's does.
+func copy() -> ShipAction:
+	var c := ShipAction.new()
+	c.thrust = thrust
+	c.rotation = rotation
+	c.turbo = turbo
+	return c