docs/storage-model.md still says V1 has no store-directory lock #796

What

docs/storage-model.md:858 opens the Concurrency section with:

V1 has no store-directory lock: Pointbreak does not coordinate writers with lockfiles, leases, a daemon, IPC, or filesystem notifications.

That is no longer true. A per-event authority lock exists and is taken inside the event-store write primitives (src/session/store/authority_lock.rs, STORE_AUTHORITY_LOCK_FILE = "authority.writer.lock"; acquired by record_event_once_acknowledged / record_change_event_once_acknowledged in src/session/store/event_store.rs, and held across an ingest batch by EventWriteBatch).

The rest of the paragraph is still accurate and worth keeping: the content-addressed primitives are what make concurrent writers to one store directory safe, identical events converge, different events never collide, conflicting events under one idempotency key fail loud, and no shared mutable projection file is written. The same section later says "Any store-directory lock added later must be scoped to the store directory, never 'one clone, one writer'" (:872), which now reads as future tense for something that partly exists.

Why it is not a drive-by edit

No public document describes the authority lock today: not its scope (per event write, inside the primitive, not a store-wide session lock), not what it does and does not promise to a concurrent writer, and not how it relates to the "no coordination" claim the paragraph makes. Writing an accurate replacement means deciding what to promise publicly, which belongs with the concurrent-store work rather than an incidental docs fix. A previous change touched this same paragraph for an unrelated reason and deliberately left the sentence alone rather than invent wording.

Acceptance sketch

• The Concurrency section describes the per-event authority lock truthfully: its scope, that it is not a store-wide or one-writer-per-clone lease, and that content-addressed writes remain the property that makes concurrent writers safe.
• The forward-looking sentence at :872 is reconciled with what now exists.
• No new promise is made about cross-process coordination that the implementation does not keep.