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/background_fps.gd

@@ -0,0 +1,22 @@
+extends Node
+
+# Autoload: drops Engine.max_fps while the window is unfocused, so an idle
+# background window doesn't keep rendering at whatever uncapped rate the
+# hardware can hit. Independent of — and complementary to — the per-menu
+# refresh-rate cap in main_menu.gd/settings_menu.gd, which only covers menu
+# screens; this covers every scene, including gameplay.
+
+const BACKGROUND_FPS := 30
+
+# 0 means "uncapped"; also what we restore to if focus is lost before any
+# menu/gameplay scene has had a chance to set its own cap.
+var _foreground_max_fps := 0
+
+
+func _notification(what: int) -> void:
+	match what:
+		NOTIFICATION_APPLICATION_FOCUS_OUT:
+			_foreground_max_fps = Engine.max_fps
+			Engine.max_fps = BACKGROUND_FPS
+		NOTIFICATION_APPLICATION_FOCUS_IN:
+			Engine.max_fps = _foreground_max_fps