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>

crates/punktfunk-host/src/discovery.rs

@@ -0,0 +1,65 @@
+//! mDNS advertisement of the native punktfunk/1 service so native clients auto-discover the
+//! host — the native-protocol analogue of the GameStream `_nvstream._tcp` advert
+//! ([`crate::gamestream::mdns`]).
+//!
+//! The service type is **`_punktfunk._udp.local.`** (UDP because punktfunk/1 is QUIC, and the
+//! advertised port is the QUIC control/data port a client `--connect`s). TXT records carry:
+//! - `proto` — the wire protocol id ([`NATIVE_PROTO`]), so a future incompatible revision is
+//!   distinguishable by discovery alone;
+//! - `fp` — the host certificate SHA-256 (lowercase hex), the exact value a client pins. mDNS is
+//!   unauthenticated, so this is advisory — TOFU/pinning still verifies it on connect — but it
+//!   lets a picker show the fingerprint and pre-pin a chosen host;
+//! - `pair` — `required` or `optional`, so a client can tell up front whether it must run the PIN
+//!   pairing ceremony before it can stream;
+//! - `id` — the stable host uniqueid (dedup across IPs / re-advertises).
+
+use anyhow::{Context, Result};
+use mdns_sd::{ServiceDaemon, ServiceInfo};
+use std::collections::HashMap;
+use std::net::IpAddr;
+
+/// The native-protocol mDNS service type. Clients browse this to find punktfunk/1 hosts.
+pub const NATIVE_SERVICE: &str = "_punktfunk._udp.local.";
+
+/// Wire protocol id advertised in the `proto` TXT record.
+pub const NATIVE_PROTO: &str = "punktfunk/1";
+
+/// Holds the mDNS daemon; dropping it unregisters the service.
+pub struct Advert {
+    _daemon: ServiceDaemon,
+}
+
+/// Advertise the native host on the LAN. `fingerprint` is the host cert SHA-256 (lowercase hex);
+/// `require_pairing` tells a discovering client whether it must pair before it can stream.
+pub fn advertise_native(
+    hostname: &str,
+    ip: IpAddr,
+    port: u16,
+    fingerprint: &str,
+    require_pairing: bool,
+    uniqueid: &str,
+) -> Result<Advert> {
+    let daemon = ServiceDaemon::new().context("create mDNS daemon")?;
+    let host_name = format!("{hostname}.local.");
+    let mut props: HashMap<String, String> = HashMap::new();
+    props.insert("proto".into(), NATIVE_PROTO.into());
+    props.insert("fp".into(), fingerprint.to_string());
+    props.insert(
+        "pair".into(),
+        if require_pairing { "required" } else { "optional" }.into(),
+    );
+    props.insert("id".into(), uniqueid.to_string());
+    let service = ServiceInfo::new(NATIVE_SERVICE, hostname, &host_name, ip, port, props)
+        .context("build native mDNS ServiceInfo")?;
+    daemon
+        .register(service)
+        .context("register native mDNS service")?;
+    tracing::info!(
+        service = "_punktfunk._udp",
+        port,
+        host = %host_name,
+        pair = if require_pairing { "required" } else { "optional" },
+        "native punktfunk/1 mDNS advertising"
+    );
+    Ok(Advert { _daemon: daemon })
+}