fix(render #53): incomplete-entity flag must persist across same-entity tuples

User reported (cache enabled, post-c55acdc): drudge statue renders fully
but many trees are missing branches. Cache-disabled A/B run rendered trees
correctly. So the bug is in the cache wiring.

Root cause: c55acdc's `currentEntityIncomplete = false;` reset fired
UNCONDITIONALLY at the top of every iteration. For a tree with MeshRefs
[trunk valid, branches null, leaves valid], the tuple sequence is:

  - tuple 0 (trunk): no flag set
  - tuple 1 (branches): TryGetRenderData null → set flag, continue
  - tuple 2 (leaves): unconditional reset → flag = false (WRONG)
  - end-of-entity: flag is false, scratch has trunk+leaves batches but NOT
    branches → MaybeFlushOnEntityChange populates a PARTIAL cache entry
  - cache hits forever serve trunk+leaves with no branches

Drudge happened to render correctly because its missing MeshRef was at the
END of its MeshRefs list — no later tuple reset the flag.

Adds a per-tuple `prevTupleEntityId` tracker for entity-change detection,
updated UNCONDITIONALLY at end of each tuple (including tuples that skip
via null renderData). The flag-reset block now fires ONLY on actual entity
change. Within the same entity, the flag accumulates across tuples.

Also includes ACDREAM_DISABLE_TIER1_CACHE=1 diagnostic env-var added
inline (was stashed previously) for future A/B testing.

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

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

@@ -74,6 +74,11 @@ public sealed unsafe class WbDrawDispatcher : IDisposable
     // miss-populate / hit-fast-path through the loop.
     private readonly EntityClassificationCache _cache;
 
+    // ACDREAM_DISABLE_TIER1_CACHE=1 A/B diagnostic — forces every static
+    // entity through the slow path. Read once in ctor.
+    private readonly bool _tier1CacheDisabled =
+        string.Equals(Environment.GetEnvironmentVariable("ACDREAM_DISABLE_TIER1_CACHE"), "1", StringComparison.Ordinal);
+
     /// <summary>
     /// A.5 T22.5: gate for GL_SAMPLE_ALPHA_TO_COVERAGE around the opaque pass.
     /// Default true matches T20 behavior. Set false for Low/Medium presets that
@@ -423,6 +428,14 @@ public sealed unsafe class WbDrawDispatcher : IDisposable
         // some parts missing permanently. Reset on entity change.
         bool currentEntityIncomplete = false;
 
+        // Per-tuple entity tracker used purely for entity-change detection.
+        // Updated UNCONDITIONALLY at end of every tuple (including tuples that
+        // skip via null renderData), so the flag-reset block below correctly
+        // distinguishes "new entity" from "same entity, different tuple."
+        // populateEntityId can't be used for this because it's only set after
+        // a successful slow-path classification.
+        uint? prevTupleEntityId = null;
+
         foreach (var (entity, partIdx, landblockId) in _walkScratch)
         {
             if (diag) _entitiesSeen++;
@@ -454,12 +467,26 @@ public sealed unsafe class WbDrawDispatcher : IDisposable
             // the tracker so MaybeFlushOnEntityChange sees the cleaned state
             // and no-ops for this entity. Reset the incomplete flag for the
             // new entity so each one gets a fresh measurement.
-            if (populateEntityId.HasValue && populateEntityId.Value != entity.Id && currentEntityIncomplete)
+            //
+            // CRITICAL: the flag reset must fire ONLY on entity change, not
+            // every tuple. Resetting per-tuple within the same entity would
+            // undo a null-renderData flag set by a previous tuple of the same
+            // entity → if the missing MeshRef sits in the MIDDLE of the
+            // entity's MeshRefs list, a later valid tuple's reset would
+            // re-mark the entity "complete" and let partial data populate
+            // the cache. Trees with [trunk valid, branches null, leaves
+            // valid] hit this exactly — branches never recover.
+            bool isNewEntity = !prevTupleEntityId.HasValue || prevTupleEntityId.Value != entity.Id;
+            if (isNewEntity)
+            {
+                if (populateEntityId.HasValue && currentEntityIncomplete)
                 {
                     _populateScratch.Clear();
                     populateEntityId = null;
                 }
                 currentEntityIncomplete = false;
+            }
+            prevTupleEntityId = entity.Id;
 
             // Flush-on-entity-change: if the previous entity accumulated any
             // batches AND this iteration is for a different entity, populate
@@ -489,7 +516,7 @@ public sealed unsafe class WbDrawDispatcher : IDisposable
             // ApplyCacheHit, sets lastHitEntityId, and continues. Subsequent
             // tuples of the same entity short-circuit at the top of the loop
             // body via the lastHitEntityId == entity.Id check above.
-            if (!isAnimated && _cache.TryGet(entity.Id, landblockId, out var cachedEntry))
+            if (!isAnimated && !_tier1CacheDisabled && _cache.TryGet(entity.Id, landblockId, out var cachedEntry))
             {
                 ApplyCacheHit(cachedEntry!, entityWorld, AppendInstanceToGroup);