The pimdir backend shows a short public id, not the link id

The pimdir backend set Envelope.id = link_id (a long mid:…@… string) that filled the envelope table's ID column and forced users to handle it. io-pimdir now assigns each item a per-collection public seq; the backend uses it:

• envelope_from_item sets id = seq.to_string().
• get_message and synced_placement (the write ops' resolver) parse the id as a seq and resolve it to the internal link_id via store.get_item(collection, seq) before reading the body / staging the mutation. parse_id fails clearly on a non-numeric id.
• add_message returns the new item's seq via store.seq_for_link.

Verified against a live neverest-synced Stalwart store: envelope list shows ids 1..N (fitting the table), message read 3 reads by the short id, flag add 2 --flag flagged resolves the id and stages the mutation, and a non-numeric id gives "Invalid message id … (expected a number)".

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