discovery: add opt-in mDNS LAN discovery

Add scoped mDNS / DNS-SD discovery for peers on the same local link,
giving sub-second pairing without a relay or NAT-traversal roundtrip.
A node advertises its npub, protocol version, and an optional network
scope over link-local multicast, and browses for matching adverts to
initiate Noise handshakes against same-LAN peers.

LAN discovery is disabled by default; operators enable it with
node.discovery.lan.enabled: true. Default-off avoids reintroducing a
per-LAN identity broadcast on nodes that have deliberately disabled
other discovery channels, and avoids any multicast surprise on upgrade.

The startup advertised-port picker now excludes bootstrap transports
and selects a non-bootstrap operational UDP transport with a stable
lowest-id selector, so the advertised port is deterministic across
restarts rather than dependent on HashMap iteration order. This
matches the per-dial transport selection used for discovered peers.

Co-authored-by: Johnathan Corgan <johnathan@corganlabs.com>

Cargo.toml

@@ -29,6 +29,7 @@ tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 tokio = { version = "1", features = ["rt", "macros", "signal", "sync", "net", "time", "process", "io-util"] }
 futures = "0.3"
 simple-dns = "0.11.2"
+mdns-sd = "0.19"
 socket2 = { version = "0.6.2", features = ["all"] }
 tokio-socks = "0.5"
 portable-atomic = { version = "1", features = ["std"] }