diag+docs: confirm grey-BG root cause = unfilled slot-table (label_125bd via stubbed coroutine op 0x140)

Traced end-to-end (AGE_DIAG_SETTEX): set-texture slot=G[0x62452] <- query-gfx-object?
(-1 for unregistered CG handles) -> fallback lookup-array-2d(rec[s3]=G[0x3239])=0
because the slot table is never filled: label_125bd (SC0000 0x50f, slots 4..13) is
gated behind the scene-coroutine framework (0x140 coroutine-yield, stubbed). Adds
env-gated VM set-texture/query slot diagnostics + GfxState.IsRegistered.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

engine/Age.Engine/Model/GfxState.cs

@@ -113,6 +113,7 @@ public sealed class GfxState
     /// <summary>Op 0x215 (query-gfx-object): native returns std::map::find(handle) — the registered value (=handle),
     /// or 0xffffffff (=-1) when the handle was never 0x1a2-registered. NOT a fabricated slot allocator.</summary>
     public int QuerySlot(long handle) => _registry.Contains(handle) ? (int)handle : -1;
+    public bool IsRegistered(long handle) { lock (_lock) { return _registry.Contains(handle); } }
     public long QueryField(long idx) => _fieldTable.TryGetValue(idx, out var v) ? v : 0;
 
     public void Release(long handle)