wip(assembler): round S8 -- region rebuilds no longer stall admission; result backlog capped

voxel.CoreRegionConcurrentAdmission (default on): tiles independent of the
entering region keep submitting while a region page compiles; pages are
immutable shared_ptr owners so running readers survive eviction. 6 m/s: 11
whole-pass skips -> 0, walkable ring ready at every centre change.
voxel.StreamingResultBacklogCap=256 bounds finished results (slow-apply worst
case 125 -> 76 MB). Spawn unchanged at ~7.7 s: LOD8 is 46% of worker time.
At 60 m/s + teleport the collision ring is outrun (102/113 centres).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmSoF64BbRFmk9C3p1VsBJ

ARCHITECTURE.md

@@ -584,6 +584,13 @@ driven by `EditorBrush*` props.
   admission count. `voxel.StreamingDynamicNearReserve=1` holds the authored near reserve while moving
   or before the walkable gate, then releases it to coarse work after the stationary ring is ready.
   Stationary player ≈ free. (Old per-frame O(loaded×desired) scan = 22ms.)
+  Core region compilation is a separate resource gate. With the default
+  `voxel.CoreRegionConcurrentAdmission=1`, tile keys that need the pending region are withheld, while
+  independent keys may use remaining worker slots. `0` restores the all-admission wait. Region pages
+  are built off the lookup map, published as immutable shared pages, and reclaimed when the final
+  reader releases its shared owner. `voxel.StreamingResultBacklogCap=256` independently bounds queued
+  result payloads plus active tile tasks and held prequeue results; `0` disables this memory backstop.
+  Result count is not used as a worker-slot count.
 - **LOD** changes HOT-SWAP (`LoadTile` only, never unload-first) → no holes. LOD
   reconciliation lives in the PERSISTENT per-frame submit loop (same loop as new-chunk loads),
   NOT as a one-shot on the boundary-cross frame — a one-shot drops every chunk past the task
@@ -1565,6 +1572,43 @@ the recipe version remains unchanged. Null-RHI runs do not prove player-visible
 [`Standalone/Research/WorldAssemblerM3/Round11/REPORT.md`](Standalone/Research/WorldAssemblerM3/Round11/REPORT.md)
 for the audit counts, RMC review, quality matrix, commands and binary hashes.
 
+### 8.29 Concurrent region admission and bounded result backlog — World Assembler S8 / M3 Round 12
+
+The S7 moving trace found `bCoreRegionCompilePending` returning before all tile submissions. Each
+such update could leave most slots unused while a new player center requested a region page. The
+default `voxel.CoreRegionConcurrentAdmission=1` lets `LoadCoreTile` continue for keys outside the
+pending and in-flight region sets. Keys that need those sets wait for publication; setting the CVar
+to `0` restores the old all-admission gate for an A/B control. This changes scheduling only.
+
+`World::compileTileRegion` builds region pages without mutating pages already in use, then publishes
+them under the region-map lock. `generateTile` copies a `shared_ptr` while holding the map's shared
+lock and keeps it for the whole mesh read. Evicting or replacing a map entry therefore cannot free a
+page under an existing worker. The actor's shared `WorldLock` prevents a world edit/reset while a
+tile reader or region compiler is active; `EndPlay` waits for the shared active-task count before
+destroying the actor's UObject state.
+
+The result backlog has its own backpressure control. `voxel.StreamingResultBacklogCap=256` counts
+queued MPSC results, active tile-generation tasks and held visible-LOD0 prequeue results. Reaching
+the cap stops new tile admissions until apply drains results or active tasks retire. It does not make
+finished results consume worker slots in the normal admission count. `0` is the uncapped negative
+control; the Core-off legacy test also exercises the default cap.
+
+On the 25 cm owner SurfaceWorld plan, the matched 6 m/s CVar-off/on runs both completed 11 region
+requests (one initial desired set plus ten center moves). The old gate recorded 11 global scheduler
+skips; concurrent admission recorded none and submitted 110 unrelated tiles with no dependent-key
+declines. All eleven changing support centers had the nine-tile walkable ring ready immediately.
+Accumulated pending windows were about 0.43 s, but measured compiler wall time totaled about 11 ms.
+At 60 m/s with a 400 m teleport, the trace recorded 113 region requests, 782 independent submissions
+and 983 dependent-key declines. Most support centers advanced before their nine-tile ring could
+become ready, so the high-speed gate still fails its travel-time target.
+
+The shared result trace now records `queued_peak`, peak payload bytes, the configurable cap, and cap
+stops. A slow-apply teleport stress run peaked at 256 results / 76.2 MB with the default cap; the
+uncapped control reached 1,577 / 125.1 MB. Payload bytes exclude queue-node and allocator overhead.
+The cap kept the observed backlog finite without reinstating the S6/S7 finished-result worker-slot
+decline. Full timings, per-center records, quality checks and hashes are in
+[`Standalone/Research/WorldAssemblerM3/Round12/REPORT.md`](Standalone/Research/WorldAssemblerM3/Round12/REPORT.md).
+
 ## 9. Multiplayer model (listen-server first, dedicated-friendly)
 
 > **Status: transport is DESIGN ONLY — nothing is networked in-tree yet** (no `Replicated`/`HasAuthority`/RPCs; a