feat: LAN-direct — Locator seam + QUIC + mDNS (C1 + C2)

[frahlg]

Implements docs/superpowers/specs/2026-06-13-c1-c2-lan-direct-locator-design.md
(approved 2026-06-13). CLI-only, Go-only — the browser keeps using the relay; no web
changes, no byte-identical crypto gate touched.

Stacked on #41 (B1.4+B1.5). It reuses ownerPubFromBinding, the cached wallet
binding, and the base58 owner_id. Base is b1.4-b1.5-wallet-wire; once #41 merges to
main I'll rebase this onto main and retarget.

What this delivers

mir attach reaches a mir up node on the same LAN with no relay — zero-config mDNS
discovery + a direct QUIC transport — by inserting a Locator seam under the unchanged
Noise-KK session. The relay/WAN path is untouched.

How it works

• C1 — Locator seam (pure refactor). A Locator (package client) turns a Machine
  into a live peer.MsgConn; Attach composes [LAN, relay] and runs Noise-KK over the
  first that connects. Today's relay path moves verbatim into relayLocator. Relay e2e
  stays green (behavior-preserving).
• C2 — LAN-direct (LANLocator).
  • Discovery: mir up advertises _miranda._udp (instance = machine_id); the client
    browses + matches. Discovery yields only an address — never trust.
  • Transport: QUIC with an ephemeral self-signed cert + skip-verify; the real auth
    is Noise-KK + the wallet binding that runs inside the stream (internal/quicmsg, a
    length-framed peer.MsgConn).
  • Wire: the wallet binding is frame 0; the agent checks IsOwnerPinned +
    VerifyBinding (reusing ownerPubFromBinding), pins binding.x25519, then runs the
    same authenticated PTY session as the relay path (serveAuthenticated).
  • Default + escapes: LAN-first with a ~600 ms budget then relay fallback;
    mir attach --relay-only and mir up --no-lan.

Trust (unchanged) + new surface

Locate but never impersonate: Noise-KK pins host_pub, the agent pins the wallet. mDNS
spoofing / a rogue LAN host yields at worst a failed handshake (DoS) — never
impersonation or plaintext. New exposure: the agent now accepts inbound LAN connections
(bounded by the same pre-auth limiter) and mDNS reveals an opaque machine_id. Both off
with --no-lan. Documented in SECURITY.md.

QUIC-over-WAN is the destination, not this step

WAN's hard part is NAT traversal (ICE), which QUIC alone doesn't solve; WebRTC already does
and ships. A future QUICHolePunchLocator (DCUtR + circuit-relay, north-star C4) drops into
the same seam — captured as a stated future, not built.

Tests

• internal/quicmsg: length-framed round-trip (incl. 70 KB frame) + ctx-cancel.
• client: lanLocator sends frame0 binding; ErrUnreachable on miss/no-wallet;
  attachLocators composition; dialFirst fallthrough; skippable live-mDNS resolve.
• agent: real-shell echo over QUIC, no relay; unpinned binding rejected pre-Noise;
  relay e2e still green after the serveAuthenticated extraction.
• go test ./..., go vet, gofmt clean. Real mir binary exposes --relay-only /
  --no-lan.

New deps (non-crypto, Go-only)

github.com/quic-go/quic-go v0.60.0, github.com/grandcat/zeroconf v1.0.0.

Follow-up (not in this PR)

• deploy/netsim LAN path (cross-container mDNS); the Go tests already prove the wire.
• happy-eyeballs race to remove the ~600 ms LAN-miss penalty (kept sequential here to avoid
  adding concurrency to the working relay path).

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