feat(discovery): native-protocol LAN auto-discovery over mDNS

Both the unified host (serve --native) and standalone m3-host now advertise the
native punktfunk/1 service over mDNS (_punktfunk._udp) — the analogue of the
GameStream _nvstream._tcp advert. TXT records carry proto, the host cert
fingerprint (fp, the value clients pin), the pairing requirement
(pair=required|optional), and the host id. New crate::discovery module, wired
into m3::serve so both host entry points get it; best-effort, never blocks
streaming (--connect always works).

Client gains `punktfunk-client-rs --discover [SECS]`: browses the LAN and prints
each host (name, addr:port, pairing, fingerprint), then exits. Apple clients
browse the same service natively via NWBrowser (service type + TXT keys are the
contract).

Validated cross-LAN: the dev box discovered the GNOME-box appliance
(pair=required) and a standalone synthetic host (pair=optional); fingerprint and
pairing state correct in both.

Also refresh the now-stale sendmmsg caveat in the bitrate doc (batched/paced send
landed + validated to 1 Gbps) and mark the encode|send thread split done in §12.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -43,6 +43,10 @@ Low-latency desktop/game streaming stack, Linux-first, with a shared Rust protoc
   no offline dictionary attack) establishes mutual trust:
   clients present persistent identities via QUIC client auth, the host stores paired
   fingerprints (`punktfunk1-paired.json`) and can gate sessions with `--require-pairing`.
+  **LAN auto-discovery**: both `serve --native` and `m3-host` advertise the native service over
+  mDNS (`_punktfunk._udp`, `crate::discovery`) with TXT `proto`/`fp`(cert fingerprint to
+  pin)/`pair`(required|optional)/`id`; `punktfunk-client-rs --discover` lists hosts, Apple clients
+  browse the same service via NWBrowser (validated cross-LAN 2026-06-12).
   **Mid-stream mode renegotiation**: `Reconfigure` on the still-open control stream — the
   host rebuilds output+encoder at the new mode in ~90 ms while the data plane runs on
   (validated live: one .h265 with 720p and 1080p segments). Measured on-box at 720p120: 1680/1680 frames, **p50 0.83 ms**