perf(rendering): retain synchronous frame scratch

Reuse the PView frame input, publish mutation-invalidated landblock views, and avoid constructing optional shadow iterators while preserving title and lifecycle visibility facts.

docs/plans/2026-07-25-modern-runtime-slice-h.md

@@ -80,21 +80,26 @@ subtree-withdrawal suites preserve exact event and resource counts.
 Landed evidence:
 [`../research/2026-07-25-slice-h-a3-attached-pose-reconcile.md`](../research/2026-07-25-slice-h-a3-attached-pose-reconcile.md).
 
-### H-a4 — diagnostics and frame scratch
+### H-a4 — diagnostics and frame scratch — COMPLETE
 
 1. Reuse one synchronous `RetailPViewFrameInput` owned by
    `WorldSceneRenderer`; reset every field before each draw.
 2. Expose stable borrowed landblock entry/bounds views instead of creating
    iterator state in each frame.
-3. Consumer-gate diagnostic landblock visibility counting and shadow
-   enumeration. When no DebugVM/collision/probe consumer exists, publish a
-   neutral diagnostic outcome without scanning resident landblocks.
+3. Consumer-gate optional shadow enumeration. Preserve landblock visibility
+   counting because the normal title and lifecycle checkpoints are active
+   consumers of `lb V/T`; neutralizing it would be a visible/automation
+   behavior change.
 4. Do not rework `InteriorEntityPartition` while the G4 referee remains.
    Issue #241 closes with G5 old-route deletion, not by editing the fallback.
 5. Profile capped and uncapped production paths and retain evidence.
 
 Gate: identical framebuffer/scene digest, no per-frame PView input allocation,
-and no diagnostic world traversal with all consumers disabled.
+no iterator allocation for stable landblock views, and no shadow traversal
+with all optional consumers disabled.
+
+Landed evidence:
+[`../research/2026-07-25-slice-h-a4-frame-scratch.md`](../research/2026-07-25-slice-h-a4-frame-scratch.md).
 
 ## 3. H-b — exact light top-k
 

docs/research/2026-07-25-slice-h-a4-frame-scratch.md

@@ -0,0 +1,69 @@
+# Slice H-a4 — retained frame scratch and diagnostic traversal
+
+## Scope
+
+This is a behavior-preserving ownership/allocation correction. It does not
+change retail visibility, PView construction, spatial ordering, collision
+debug drawing, title facts, or lifecycle checkpoint facts.
+
+## Findings
+
+Three independent steady-frame allocation sources remained:
+
+1. `WorldSceneRenderer` constructed one `RetailPViewFrameInput` reference
+   object for every PView frame even though `DrawInside` consumes it
+   synchronously.
+2. `GpuWorldState.LandblockEntries` and `LandblockBounds` were C# iterator
+   methods, so merely obtaining each sequence constructed fresh iterator
+   state. The underlying spatial membership changed only at update-thread
+   publication boundaries.
+3. `WorldSceneDiagnosticsController` passed
+   `ShadowObjectRegistry.AllEntriesForDebug()` to the DebugVM publisher even
+   when no developer UI consumer existed. The publisher correctly avoided
+   enumeration, but the yield sequence itself had already been constructed.
+
+The execution plan initially described landblock visibility counting as an
+unconsumed diagnostic. That was incorrect in the current application:
+`WorldRenderFrameOutcome` feeds the always-visible FPS/title `lb V/T` values
+and connected lifecycle snapshots. Returning a neutral count would therefore
+change observable behavior and invalidate lifecycle evidence. The count is
+retained; only its iterator allocation is removed.
+
+## Implementation
+
+- `WorldSceneRenderer` owns one `RetailPViewFrameInput` and resets every field
+  before the synchronous `DrawInside` call. The input is publicly read-only
+  and internally mutable so downstream render owners cannot change borrowed
+  frame facts.
+- `GpuWorldState` exposes retained entry, no-animated-index entry, and bounds
+  lists. They rebuild only after landblock membership, resident entity-list,
+  animated-index, or AABB changes. Existing render-traversal slot order is
+  preserved exactly.
+- Landblock diagnostic counting uses indexed access over the retained bounds
+  view.
+- Shadow enumeration is constructed only when the DebugVM is mounted. The
+  independent collision-wireframe switch keeps its existing enumeration.
+- `InteriorEntityPartition` remains untouched while the G4 referee is still
+  retained. Its removal and issue #241 close remain G5 work after the user
+  accepts the G4 visual gate.
+
+## Conformance
+
+- Stable retained render-view getters allocate zero bytes over 1,000 polls.
+- A spatial mutation refreshes the same borrowed list identity and publishes
+  the new count.
+- Two successive PView frames receive the same input object, reset with the
+  current frame values.
+- Existing quiescence, render traversal, animated-index, PView executor,
+  abort/recovery, and DebugVM tests remain unchanged in behavior.
+
+## Rollback boundary
+
+This work is independent of the G4 production-source cutover. If tomorrow's
+visual gate rejects G4, use:
+
+```text
+git revert ef1d263337997bb030eadb7b8e71d73dc659907a
+```
+
+Do not revert this Slice H-a4 commit, G3, or the portal-warmup corrections.