refactor(battlenode): guard generated iface-impl against regen + stub visibility (M-HC-0 review)

- _IfaceImpl.g.cs: extend header to warn about hand-edits; tag all bare
  // HEADLESS-FIX lines with their milestone (M13 on GetSideLogControl ×2)
  so `grep HEADLESS-FIX` reliably surfaces every block before a regen.
- HeadlessHandViewStub / HeadlessPlayQueueViewStub: narrow from public to
  internal sealed — both stubs are consumed only within SVSim.BattleEngine
  (via the generated partial impls); no public surface exposes the concrete
  type, so internal is correct and aligns with HeadlessIconAnimations.
- SessionBattleEngine.SeedMulliganInfoControl: add one-line comment on the
  GetComponent<MulliganInfoControl>() call explaining the shim's lazy
  materialisation behaviour (otherwise reads like a guaranteed NRE).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.BattleEngine/Shim/View/HeadlessHandViewStub.cs

@@ -12,7 +12,7 @@ using UnityEngine;
 
 namespace Wizard.Battle.View
 {
-    public sealed class HeadlessHandViewStub : HandViewBase
+    internal sealed class HeadlessHandViewStub : HandViewBase
     {
         // Shared instance the generated IBattlePlayerView.HandView getters return headless.
         public static readonly HeadlessHandViewStub Instance = new HeadlessHandViewStub();

SVSim.BattleEngine/Shim/View/HeadlessPlayQueueViewStub.cs

@@ -16,7 +16,7 @@ using Wizard.Battle.View.Vfx;
 
 namespace Wizard.Battle.View
 {
-    public sealed class HeadlessPlayQueueViewStub : PlayQueueViewBase
+    internal sealed class HeadlessPlayQueueViewStub : PlayQueueViewBase
     {
         // Shared instance the generated IBattlePlayerView.PlayQueueView getters return headless.
         public static readonly HeadlessPlayQueueViewStub Instance = new HeadlessPlayQueueViewStub();