docs(render #53): clarify DebugCrossCheck's wiring status

Code review of f16604b flagged that DebugCrossCheck's XML doc claimed
"called once per static-entity cache hit per frame" — overstated. The
method is currently exercised by unit tests only; the dispatcher's
cache-hit branch fires a simpler predicate assert (!isAnimated) at
production hit time, not the full live-state cross-check. Wiring the
full cross-check is the spec section 6.5 stretch goal, kept open as a
follow-up.

Doc-only change. No behavior change. 1708 / 8 baseline preserved.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/Wb/EntityClassificationCache.cs

@@ -113,8 +113,12 @@ internal sealed class EntityClassificationCache
     /// </para>
     ///
     /// <para>
-    /// Zero cost in Release. In DEBUG, called once per static-entity cache
-    /// hit per frame — adds modest overhead. Acceptable for dev runs.
+    /// As of Phase 4 (commit f16604b) this method is exercised by unit tests
+    /// only; the dispatcher's cache-hit branch fires a simpler predicate assert
+    /// (<c>!isAnimated</c>) at production hit time. Wiring the full live-state
+    /// cross-check into the per-entity branch is the spec section 6.5 stretch
+    /// goal and remains open as a follow-up. Zero cost in Release; the method
+    /// stays here so the regression-class guard is locked behind tests.
     /// </para>
     /// </summary>
     public void DebugCrossCheck(uint entityId, IReadOnlyList<CachedBatch> liveBatches)