docs: registry/storage accuracy notes + surface-freeze checklist step

[johnleider]

What

Documentation-accuracy fixes, each confirmed against source — no behavior changes (docs).

• createRegistry (RegistryTicket.index JSDoc) — notes the lazy-reindex contract: a ticket read via values()/entries() immediately after unregister()/offboard() may carry a stale index until the next position-reading call (register/upsert/browse/lookup/move/reorder/seek) drains the deferred reindex.
• useStorage (StorageContext.remove/clear JSDoc) — tightened to reflect actual scope: they act on keys accessed through this storage instance; keys persisted by a previous session but never read here are not removed. Drops the "all keys from storage" overstatement.
• createRegistry docs — new FAQ entry: per-item teardown is O(n²) (each unregister is an O(n) locate+splice), so batch removals via offboard/clear and bound the mounted/registered count with createVirtual for large lists.
• new-feature-checklist — adds the manual step to update packages/0/src/surface.test.ts (the public-surface freeze test) when a new public export is added — both tables + the bottom checklist.

Verification

pnpm typecheck:0 + pnpm build:docs — both green.

The checklist step references packages/0/src/surface.test.ts, which is added in #409. Merge #409 first (or together) so the referenced file exists on master.

[github-actions]

⚠️ No changeset found

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pnpm changeset

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