feat: multi-peer BEACON file swarm (chunk bitmap + parallel fetch)

[krwg]

Summary

Move large LAN file distribution beyond one-to-one TCP seeding toward BitTorrent-like swarming inside a group: chunk hashes, have-bitmaps, parallel requests from multiple peers.

Building blocks already present

• beacon-seed-file.js, beacon-mesh.js, file-tcp-send.js
• tcp-framing.js / tcp-write-queue.js

Design sketch

1. Split files into fixed-size chunks with per-chunk hashes (resume/verify)
2. Announce not only “have file X” but chunk bitmap
3. Receivers pull missing chunks from multiple group peers in parallel
4. Benefit: group of N on LAN uses aggregate bandwidth, not one uploader’s uplink

Scope note

Large protocol feature — expect design doc + staged rollout (announce bitmap → multi-source fetch → UI progress).

Acceptance

• Design doc for wire types / bitmap
• Parallel multi-peer chunk fetch for at least one group scenario
• Integrity via chunk hashes; resume-safe