docs: design for mail arrival notifications

One notification per account per arriving batch, from a new
mail-notify.sh sibling to waybar-mail.sh.

Arrival is detected with notmuch's lastmod revision counter rather
than a count delta or a date watermark: a commit is not the same as
new mail, since reading and tagging also commit, and date: is the
message's own Date header, so backdated mail would never notify and
future-dated mail would notify forever.

A separate process rather than extending waybar-mail.sh, which
already has the arrival edge: waybar owns that process, so a bar
restart would stop notifications with nothing reporting it.

State carries the database UUID beside the revision, because notmuch
revisions are only comparable within one database. Missing, corrupt
or mismatched state seeds silently: with no floor, lastmod:0..
matches all 101 unread messages here and startup becomes a wall of
popups.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017ZwWrCEbdKzmisfg9bb1nS

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