EMAIL-WP-0005-T01: add Maildir mailbox source

Adds MaildirMailboxSource reading new/ and cur/, wired through
source.maildir_dir and mailbox.protocol: maildir. Message identity is the
Maildir unique name without its :2,FLAGS suffix, so it survives the new/ to
cur/ move; it is carried on MailboxSourceMessage.dedup_uid and appended to the
message dedup key only when set, leaving fixture and IMAP keys unchanged.

Cursor ordering parses the Maildir delivery time instead of comparing names
lexically. mark_seen and a missing directory are rejected, matching the
read-only IMAP contract.

Also fixes the parse-failure path, which keyed identity on raw_message_ref.
A new/ to cur/ move rewrites that ref, so an unparseable message re-registered
as new on every rescan; it now prefers the source uid when one exists.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

config/mailbox.example.yml

@@ -10,6 +10,7 @@ mailbox:
 
 source:
   fixture_dir: tests/fixtures/mailbox
+  maildir_dir: null
 
 scan:
   mode: incremental