add Announce API for GossipSub message announcement

Implements a new Announce API that allows advertising messages via IHAVE
without pushing them to the mesh. These messages are retained in the
mcache until, at least, the specified deadline.

We send the IHAVE immediately to all connected topic subscribers,
whether in mesh, in gossip, in a cached fanout, or none of these active
pubsub states.

This enables pull-based message distribution, useful for scenarios
outside of the app's critical path, such as backup availability.
Subscribers can pull messages on-demand via IWANT requests.

Further details:

- Add Topic.Announce() method that sends IHAVE gossip to topic
  subscribers with expiry-based message retention
- Refactor MessageCache to support dual storage model:
  - Sliding window for regular published messages
  - Time wheel for announced messages with TTL-based expiry
- Add GossipSubAnnouncementMaxTTL parameter (default 60s) for sizing
  announcement storage
- Rename MessageCache methods for clarity (Put→AppendWindow,
  Shift→ShiftWindow, GetGossipIDs→GossipForTopic). Add missing godocs.
- Implement unified message storage with reference counting to handle
  messages in both window and announcement wheel
- Add heartbeat cleanup for expired announcements via PruneAnns()
- Add comprehensive test coverage for announcement functionality
  including storage, delivery, expiry, duplicates, and edge cases

Author: raulk