The pimdir public id is message-scoped (spec wording follow-up)

Follow-up to pimdir-public-id: io-pimdir's seq changed from per-collection to message-scoped and store-global (message-scoped-seq there), so the same message now shows the same id in every mailbox it is filed in. The backend needs no code change — it already sets Envelope.id = item.seq and resolves it via get_item(collection, seq) — but the spec wording said "per-collection". Corrected the backends requirement to describe the id as a store-assigned integer, the same across every mailbox a message is filed in.

Verified live: a message filed in INBOX and Archive shows id 1 in both, and message read 1 -m Archive reads it.

Spec updated: backends (MODIFIED "pimdir shows a short public id" wording).