fix(UXAssist): centralize mod-feature lifecycle to prevent duplicate execution

ModFeatureRegistry is a static class with shared collections accumulating
features from all mods. Dependent mods (CheatEnabler, UniverseGenTweaks)
each independently called InitAll/StartAll/OnInputUpdateAll/OnUpdateAll/
UninitAll, re-running lifecycle for ALL accumulated features including
other mods' — per-frame Update ran 2-3x (breaking CheatEnabler key toggles
to net no-ops), Init/Start/Uninit ran 2-3x (double RegisterExporter causing
save corruption, double SettingChanged subscriptions, double keybind
registration).

Fix:
- Init now runs eagerly at Discover/Register time (Awake-phase), preserving
  the original timing that keybind registration depends on (game's
  UIOptionWindow._OnCreate copies keybinds only after all plugins load)
- InitAll removed entirely
- StartAll/UninitAll/OnInputUpdateAll/OnUpdateAll made internal so only
  UXAssist (host, same assembly, no InternalsVisibleTo) can drive them
- Start deferred to UXAssist.Start (after all dependents' Awake complete;
  BepInEx runs all Awakes before any Start)
- Per-feature Started idempotency + per-frame Time.frameCount guards as
  defense-in-depth
- Dependent mods reduced to Discover-only in Awake

Document the Init/Start timing contract on IModFeature and
ModFeatureAttribute so future mods can rely on it.

UniverseGenTweaks/UniverseGenTweaks.cs

@@ -58,17 +58,12 @@ public class UniverseGenTweaks : BaseUnityPlugin, IModCanSave
 
         Localization.Register();
         UIConfigWindow.Init();
+        // Register features (Init runs eagerly here); UXAssist drives the deferred lifecycle (Start/Uninit/Update).
         ModFeatureRegistry.Discover(Assembly.GetExecutingAssembly());
-        ModFeatureRegistry.InitAll();
 
         I18N.Apply();
     }
 
-    private void OnDestroy()
-    {
-        ModFeatureRegistry.UninitAll();
-    }
-
     #region IModCanSave
     private const ushort ModSaveVersion = 1;