Implements announce channel delivery in router.go:
- Add isAnnounceAddress() and parseAnnounceName() helpers
- Add ErrUnknownAnnounce error variable
- Add expandAnnounce() to load AnnounceConfig from messaging.json
- Add sendToAnnounce() for bulletin board delivery (single copy, no claiming)
- Add pruneAnnounce() for retention-based message cleanup
- Integrate announce routing in Send()
Announce channels store ONE copy of each message (unlike lists which fan-out).
Messages persist until retention limit is reached, with oldest messages
pruned automatically when limit is exceeded.
Also includes address helpers (gt-pn2fq dependency).
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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