wip(assembler): round L7 -- shared column cache across workers: whole view 16-19% faster, world unchanged

SharedColumnCache.h: a world-owned, bounded 4-way second-tier cache of exact column values
(heightfield and floating islands), keyed by recipe/plan identity plus content hash, XY and island
id. Lock-free seqlock reads; set-local spin lock on misses. The FLazy thread-local tier stays
first. 83.6% of repeated column work was cross-worker; formula evaluations -60.9%.
W12 L1 view: Luna 2.165 -> 1.754 s; reviewer interleaved 2.059 -> 1.722 s. 8 m crossing new tiles
82 -> 52 ms. +66 MiB peak RSS. All hashes (whole view, per-LOD, exact queries, islands seed 0)
identical at W1/W8/W12, Clang off and GCC default. MSVC/UBT not compiled here.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CSFYgsdW8ejZSyiMf4qRSX

ARCHITECTURE.md

@@ -732,6 +732,15 @@ driven by `EditorBrush*` props.
   its default member initialization was never observable; removing that generated TLS leaf avoids
   constructor work without changing a height or density. Integer queries still read the cached
   payload, and fractional queries still use the same direct locals.
+- **Core shared column cache (L7)**: `Heightfield2D` and FloatingIslands keep their `FLazy`
+  worker-local caches as the first tier. A world-owned, fixed-size four-way table is the second
+  tier, keyed by recipe and plan identity plus content hash, exact XY, and an optional exact
+  sub-domain identity (FloatingIslands uses the island stable ID). Hits read versioned atomic
+  entries without taking a lock; misses take only their set's spin lock, recheck, compute once, and
+  publish. The tables are bounded and die with the world after its tile work has drained. This
+  deliberately adds shared reads on column-cache hits while preserving the no-lock hot-path rule;
+  S14's purely worker-local cache remains the first lookup. Keep large worker-local storage behind
+  `FLazy`, and do not make cache values or keys depend on engine objects.
 - **Sampler unknown-source audit**: the earlier plugin-side `FSurfaceColumn` constructor PCs no longer
   occur after the trivial-payload change. The largest remaining LOD0 unknown cluster in the final
   sampler raw CSV is `UnrealEditor-Core.dll + RVA 0xC080` (runtime PCs around
@@ -1797,6 +1806,23 @@ page was added: cross-worker repeats alone do not establish a net page-cache win
 immutable-page lifetime rules would add ownership and memory cost. Round 18 changed no output or
 recipe version. See [`Standalone/Research/WorldAssemblerM3/Round18/REPORT.md`](Standalone/Research/WorldAssemblerM3/Round18/REPORT.md).
 
+### 8.35 Shared Core column cache — World Assembler L7
+
+L7 measured 1.34 million repeated formula evaluations on the W12 L1 view before sharing: 83.6%
+were another worker's XY, 4.0% were same-worker repeats after eviction, and 12.0% were halo/normal
+repeats. The world-owned shared second tier cut measured formula executions by 60.9% and improved
+median whole-view-ready time by 19.0%; moving 8 m and generating 144 entering tiles improved median
+wall time by 36.1%. Peak RSS rose by about 66 MiB. The cache is a bounded exact-value table rather
+than published pages; no scheduling affinity or page prefetch was needed to clear the measured
+whole-view threshold.
+
+The thread-local first tier remains because it avoids shared atomics for local hits. The shared
+table has no lock on hits; only a miss takes a set-local spin lock. FloatingIslands' cache identity
+includes its island stable ID because distinct anchors can request the same XY with different
+column metrics. All W1/W12 L1 hashes, exact queries, and the seed-0 FloatingIslands fixture match
+the pre-L7 binary with Clang FP contraction off and GCC defaults. See
+[`Standalone/Research/WorldAssemblerM3/Linux7/REPORT.md`](Standalone/Research/WorldAssemblerM3/Linux7/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