Deliver add-to messages to all participant domains (SPEC v0.4.0)

[markmnl]

Implements add-to participant notification per the updated spec: an add-to message is sent to every participant domain of the message being added to -- the domains of from and every to address as well as the new recipients' -- so all participants learn recipients were added, not only the domains hosting the new batch.

Receiving side:

• A message with add-to is accepted when any participant (not only a recipient) belongs to this host's domain; such participant-only deliveries complete at code 11 through the existing path, which records the batch.
• An add-to whose parent is not stored is rejected with code 6 (parent not found) when this host has no recipient to deliver to; with a recipient it is treated as a full message delivery as before.
• An add-to batch already recorded is rejected with code 10 (duplicate).
• attachAddToRecipients marks rows for other domains with response code 11 so a host recording a received batch never treats those rows as its own delivery work.

Sending side:

• New msg_add_to_notify table tracks one row per (batch, participant domain) owed a notification, with the same retry/back-off state as recipient rows; a trigger wakes the sender when rows appear.
• findPendingTargets and deliverUnit deliver an add-to unit to a domain with a pending notify row even when it holds none of the batch's recipients, recording the outcome on the notify row.
• Add-to recipient rows now age against their batch's time_added rather than the message's time_sent, so batches added long after a message was sent still deliver.

Also adds a recipients_added notify channel fired when a batch is recorded (locally created or received), so API layers can push the updated recipient list to connected participants, and syncs SPEC.md from fmsg-spec.