feat(data): injectable memory allocator for the new ECS (wasm + SharedArrayBuffer)

[krisnye]

Description

Adds the ability to inject a custom memory allocator into the new ECS Database / Store factory, with off-the-shelf implementations that allocate numeric component storage into WebAssembly memory or a growable SharedArrayBuffer. This mirrors the precedent already present in old-ecs (createECS / core-ecs accept { allocator?: MemoryAllocator }), which the new typed-buffer-backed ECS previously lacked.

The point of the SharedArrayBuffer path: every numeric column becomes a fixed (offset, length) view into one shared backing buffer, so in principle that buffer can be posted to a worker once and its numeric components read/written across threads without copying — a foundation for future parallel system scheduling.

What's included

• Reuses the existing cache/memory-allocator.ts MemoryAllocator interface (allocate / refresh / release / needsRefresh) rather than inventing a parallel abstraction.
• New createSharedArrayBufferAllocator() (growable-SAB arena) + isSharedArrayBufferAllocatorSupported(); createSimpleMemoryAllocator (default, per-column buffers) and createWasmMemoryAllocator retained.
• wasm + SAB allocators now share one internal first-fit arena (free list over a single backing buffer).
• Correctness fix over the legacy wasm allocator: a detached TypedArray reports byteOffset/length as 0, so the old refresh could not recover a view's location after memory.grow. The arena now keeps a WeakMap<view, {offset,length,ctor}> so refresh rebuilds detached views correctly, and zero-fills reused blocks to match the always-zeroed semantics of the simple allocator.
• Threaded an optional allocator through createDatabase(plugin, { allocator }) → createStore(schema, { allocator }) → createCore → createArchetype → createTable → createTypedBuffer → number / struct / boolean / enum buffers. createDataView32 is now offset-aware so struct columns can be arena sub-regions.
• All new parameters are optional / additive — default behavior is byte-identical (each column owns its own SharedArrayBuffer/ArrayBuffer), and the whole monorepo typechecks.

Test evidence

New unit tests, all green (full @adobe/data suite: 3293 passed):

• cache/memory-allocator.test.ts — simple/wasm/SAB allocators, shared-arena contract, detached-view refresh after grow, zeroing of reused blocks, arena exhaustion, feature detection.
• typed-buffer/typed-buffer-allocator.test.ts — each storage buffer preserves data across a reallocating grow under wasm/SAB allocators; columns share one backing buffer.
• ecs/store/store-allocator.test.ts — createStore / createDatabase route component columns into the injected arena; correctness after growth past initial row capacity; default path stays per-column.

Repo-wide pnpm run lint and pnpm -r run typecheck both pass.

Known limitations / follow-ups

1. Subscription lifecycle: each column subscribes to needsRefresh and never unsubscribes... Resolved in e2385272: added TypedBuffer.dispose() (unsubscribe + release the arena block), wired through archetype.fromData to dispose replaced columns. See the review-response comment below.
2. Non-arena paths: .copy() (detached snapshots), the persistence codec (register-typed-buffer-codecs), and remapArchetypeColumn allocate with the default allocator, so post-load / post-migration columns are not placed in an injected arena. (Still out of scope; a dispose() lifecycle now exists to build on.)
3. Cross-worker liveness: actually surviving a main-thread column grow from a worker view (grow reallocates a column's block) is not solved here — this PR is the foundation, not the sync protocol.

Related PRs

None.

[krisnye]

Addressed the fable review findings in e2385272 — each with red/green unit coverage:

#	Finding	Fix	Test
HIGH	needsRefresh subscription leak	TypedBuffer.dispose() stores + calls the unsubscribe	dispose > unsubscribes from needsRefresh
HIGH	No arena-block release on disposal	archetype.fromData disposes replaced (non-retained) columns; dispose() releases the block	archetype-allocator-dispose.test.ts, dispose > releases the arena block
MED	Non-geometric grow storms	growArena prefers ≥ doubling the usable arena	arena growth is geometric
MED	release() foreign-view corruption	unknown views are ignored (no free-list insert)	release() ignores foreign views
MED	wasm assumes it owns whole Memory	createWasmMemoryAllocator(memory, { byteOffset }) reserves a base region	reserved base offset
LOW	Unconditional fill(0)	free blocks carry a zeroed flag; fresh-from-grow blocks skip the fill	covered by hands back zeroed memory even when reusing a freed block
LOW	mergeFreeBlocks full re-sort	sorted-insert + adjacent-neighbor merge only	arena-contract suite
LOW	capacity = 0 / zero-size cruft	zero-length allocation returns an untracked empty view	arena zero-length allocation
NIT	initial/max not alignment-validated	both bounds alignUp'd in the SAB allocator	alignment
NIT	inconsistent exhaustion errors	wasm grow now throws an actionable "exhausted" error	exhaustion

Notes:

• The two LOW internal refinements (fill(0) skip, sorted-insert merge) are non-behavioral — output is identical either way — so they're verified by the existing arena-correctness tests rather than a behavioral red/green.
• dispose() on a default-allocated buffer is a no-op (unsubscribe + release are no-ops there), so the default path is unchanged; the full suite (3315 tests) stays green.