mDNS device beacon (_waybeam-venc._tcp): zero-config LAN discovery

[snokvist]

Summary

Adds a lightweight mDNS/DNS-SD device beacon so the encoder is discoverable on the LAN with no configuration. A client (ground station / viewer) can browse _waybeam-venc._tcp, resolve the encoder, and read endpoints via GET /api/v1/config.

What's in this PR

• src/mdns_beacon.c — own-thread responder on UDP 5353 multicast; announces PTR/SRV/TXT/A for _waybeam-venc._tcp; sends goodbye on stop. Bare waybeam.local alias (default on) with RFC 6762 §8.2 conflict resolution (highest-IP tiebreak; suppression sticky until restart).
• src/mdns_wire.c — DNS wire codec, adapted from the OpenIPC herald DNS-SD helpers. Bounds-checked against malformed/truncated inbound packets (compression-pointer loop cap, per-field length guards).
• src/device_id.c — SoC die-ID read → serial-suffix instance naming; full die ID exposed read-only as device.serial via GET /api/v1/config; graceful fallback on parts with no readable die ID.
• New discovery config block (enabled, serviceType, name, bareAlias; default on), wired through defaults/parse/print/serialize.
• TXT carries only proto + version; endpoints/identity are probe-discovered via GET /api/v1/config (keeps TXT minimal).
• tests/test_mdns_beacon.c — unit suite for wire encode/parse + beacon behavior.

Verification

• make test → 1701 passed / 0 failed
• make verify → Maruko + Star6E backends cross-build clean
• Device-verified on Star6E (IMX335) — beacon browsed/resolved by a LAN client end-to-end.

Deliberately NOT included (kept in the Waybeam fork)

This is the encoder-side feature only. The broader Waybeam discovery/trust migration (cross-repo spec doc, and the S95waybeam apfpv init-script changes that depend on a Waybeam-specific wfbmode boot env) are intentionally left out of this upstream PR as they're ecosystem/deployment-specific.

Opened as a draft for review.

🤖 Generated with Claude Code

[snokvist]

Pushed a follow-up commit aligning the beacon more closely with the herald responder it derives from — two RFC 6762 conformance fixes:

• IP multicast TTL 1 → 255 (§11) — strict mDNS stacks drop TTL ≠ 255; the group stays link-local regardless.
• 20–119 ms response jitter (§6) before answering queries, coalescing a poll cycle's queries into one response.

No wire-codec change (record order / cache-flush classes / name compression / TTL=0 goodbye were already herald-compatible).

Device-verified on SigmaStar Star6E (ssc338q): captured multicast TTL = 255 (was 1), response latency measured 24–117 ms across repeated PTR queries, and service + suffixed host + bare waybeam.local alias all resolve. make test: 1701 passed, 0 failed.