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.BattleNode/Sessions/Engine/SessionBattleEngine.cs

@@ -277,7 +277,7 @@ internal sealed class SessionBattleEngine
     // (it isn't a Component, so WireComponentFields skips it). Node seed (allowed) — pure no-op view leaves.
     private static void SeedMulliganInfoControl(GameObject prefab)
     {
-        var ctrl = prefab.GetComponent<MulliganInfoControl>();
+        var ctrl = prefab.GetComponent<MulliganInfoControl>(); // Shim GameObject.GetComponent<T>() lazily materialises a no-op component — not a real Unity scene; this is intentional and will not NRE.
         var partsType = typeof(MulliganInfoControl)
             .GetNestedType("MulliganParts", BindingFlags.NonPublic)
             ?? throw new InvalidOperationException("MulliganInfoControl.MulliganParts nested type not found");