docs: implementation plan for mail arrival notifications

Six tasks, TDD against one bash check rather than a framework: the
script is sourced as a library so the pure functions, the ones that
only move text around, are asserted without notmuch, dunstify or
inotify in the picture.

The two parsing traps that already cost this component a debugging
session each get a fixture apiece: a key containing dots, and a
folder value containing a bracket.

Verification does not wait for mail. Backdating the stored revision
by 2000 replays a real window on demand, and the seed run before it
proves the anti-storm rule with 101 unread messages on disk.

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

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