wip(assembler): round L4 -- lazy per-thread caches: 17.3 MiB -> 43 KiB per thread, view 7% faster

VoxelForgeCoreThreadLocal::FLazy<T> (new public header ThreadLocalStorage.h) makes every large
thread_local cache a pointer that heap-allocates on that thread's first use (still worker-private,
lock-free). Static TLS 17.33 MiB -> 43 KiB (initialised image 5.8 MiB -> 21 KiB); worker start
5.2 -> 0.12 ms; idle worker RSS 17.7 MiB -> 52 KiB. Linux workers back to 8 MiB stacks.
World unchanged (L1 hashes Clang off/GCC default, W1/W12; FloatingIslands seed 0).
Reviewer interleaved W12: 2.256 -> 2.091 s. MSVC/UBT not compiled here.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ED33VnqepD4YY6gDAAA832

ARCHITECTURE.md

@@ -648,6 +648,7 @@ driven by `EditorBrush*` props.
   two global settings through `FVoxelOpContext` once per chunk, while `ClassifyBox`/`ClassifyTile`
   share the radial proof. This adds no cache, state, RNG, or clipmap stream, and does not touch the
   density grid's two-pass MC loop or any existing `thread_local` cache.
+- **Large `thread_local` caches are lazy** (`VoxelForgeCoreThreadLocal::FLazy<T>`, `Standalone/Core/include/VoxelForgeCore/ThreadLocalStorage.h`, round Linux4): the per-thread handle is a pointer; the table is heap-allocated on that thread's first use and freed at thread exit. Still worker-private and lock-free. It keeps the static TLS block at ~43 KiB instead of 17.3 MiB (glibc carves static TLS out of each thread's stack, and a Linux server pays it on every thread). New cache tables over a few KiB go through `FLazy`, never a bare `thread_local` array.
 - **Biome cache** (`ResolveBiomeSampleAt`/`FChunkBiomeCache`, §8.14): validity is a world-XY BOX +
   ChunkZ + Seed, NOT a chunk key — same reason as the SDF cache. The cell classification is
   noise-heavy; a chunk-key would thrash it on gradient-normal / +X/+Y boundary samples. Keep