refactor(core): bind generator fields once per resolved strate (L40, migration step 2); research R7

SurfaceWorld, FloatingIslands and legacy evaluators sit behind one immutable densityAt adapter held in
a variant (no virtual call per sample); BindResolvedStratumFields builds a sorted strate table at world
setup, so DensityAt, native SW/FI sampling and ClassifyBoxDetailed no longer ask the strate manager per
sample. No arithmetic change: owner hashes identical to L37/L39 at W1/W12. Cost vs L39: FI -0.66%,
SW -1.31% (noise). verify fast/medium/full PASS (full re-run alone: 407 gates).
Research R7: disk cache design for UE5/Windows/multiplayer and a zlib probe (about 1/3 size, 0.1 ms/tile).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016dhxM43dnWvUu2B5Ph1iG1

ARCHITECTURE.md

@@ -2162,6 +2162,28 @@ recipe/style audits, route support, open-air, strict layout, legacy validators,
 paired-cost results are in
 [`Standalone/Research/WorldAssemblerM3/Linux36/REPORT.md`](Standalone/Research/WorldAssemblerM3/Linux36/REPORT.md).
 
+### 8.50 Immutable field binding — World Assembler M3 L40
+
+`FStandaloneShapeWorld` now keeps a sorted `FBoundStratumField` table built from the resolved layout
+at world construction. Each entry owns a concrete SurfaceWorld, FloatingIslands, or legacy adapter
+in `FGeneratorFieldAdapterV1`, plus the resolved bounds and native plan pointer. The adapters share
+the immutable `densityAt(x,y,z,out)` shape, while `std::variant` keeps dispatch concrete instead of
+adding a virtual call to every density sample. FloatingIslands keeps its original fixed-point entry
+point for sample evaluation and its boundary parameter table is captured when the adapter is bound.
+
+`DensityAt`, native SurfaceWorld/FloatingIslands evaluation, legacy SurfaceWorld bridging and
+`ClassifyBoxDetailed` select from the bound table. The native adapters no longer ask the manager
+which generator or plan to use per sample; box proof reuses the bound field objects. The existing
+float-to-fixed conversion order, density arithmetic and shared boundary composition remain intact.
+The manager's layout-width value is captured once for the legacy-compatible chunk-index projection.
+
+The legacy adapter is still a compatibility bridge: `UVoxelGenerator::GetDensityAt` retains its
+own manager routing when invoked for legacy SurfaceWorld fields or out-of-plan fallback queries.
+Other shared-composition policy branches and passage/boundary planning remain outside this slice.
+No Unreal adapter or `VoxelWorld.cpp` change was needed, and the evaluator version is unchanged.
+L37/L39 owner hashes are the parity baseline; the L40 report records fast, medium, full and paired
+cost evidence.
+
 ## 9. Multiplayer model (listen-server first, dedicated-friendly)
 
 > **Status: transport is DESIGN ONLY — nothing is networked in-tree yet** (no `Replicated`/`HasAuthority`/RPCs; a