feat: punktfunk/1 — mid-stream mode renegotiation + PIN pairing ceremony

Renegotiation (no reconnect on resize): the handshake bi-stream stays open; the client
sends Reconfigure{mode} (typed post-handshake message), the host validates + acks
Reconfigured and rebuilds capture/encoder/virtual output at the new mode while the data
plane (keys, ports, FEC) runs untouched — the first new-mode AU is an IDR with in-band
parameter sets. NativeClient::request_mode / punktfunk_connection_request_mode; mode()
reflects the active mode. Validated live on KWin: one continuous stream, 225 frames
@1280x720 then 395 @1920x1080, ~90 ms pipeline rebuild (ffprobe shows both resolutions).

PIN pairing (mutual trust, kills TOFU MITM): clients get persistent self-signed
identities presented via QUIC client auth (generate_identity / client auth offered but
optional server-side — legacy clients still connect). Ceremony on the control stream:
PairRequest{name} → host shows a 4-digit PIN (log) + PairChallenge{salt} → client proves
with HMAC-SHA256(PIN‖salt, client_fp‖host_fp) — binding both certs means a MITM can't
forward a proof, single attempt per PIN, constant-time compare → PairResult; host
persists the fingerprint (~/.config/punktfunk/punktfunk1-paired.json), client pins the
host's. m3-host --require-pairing gates sessions on the paired set.
NativeClient::pair + punktfunk_pair/punktfunk_generate_identity in the ABI; reference
client: --pair PIN --name LABEL + auto-generated persistent identity, --remode for live
renegotiation testing. Swift wrapper: ClientIdentity/generateIdentity()/pair(),
requestMode()/currentMode(); README handoff updated.

Tested: reconfigure/pairing wire roundtrips, C-ABI mode switch ack, full in-process
ceremony (wrong PIN → Crypto, anonymous-vs-gate rejection, success → pinned session);
live wrong-PIN ceremony against the serving host (PIN logged, proof rejected).

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

crates/punktfunk-host/src/main.rs

@@ -90,6 +90,9 @@ fn real_main() -> Result<()> {
                 max_sessions: get("--max-sessions")
                     .and_then(|s| s.parse().ok())
                     .unwrap_or(0),
+                require_pairing: args.iter().any(|a| a == "--require-pairing"),
+                pairing_pin: None,
+                paired_store: None,
             })
         }
         Some("-h") | Some("--help") | Some("help") | None => {
@@ -317,6 +320,8 @@ M3-HOST OPTIONS:
     --seconds <N>                per-session stream duration, virtual source (default: 30)
     --frames <N>                 per-session frame count, synthetic source (default: 300)
     --max-sessions <N>           exit after N sessions; 0 = serve forever (default: 0)
+    --require-pairing            only serve PIN-paired clients (the host logs a 4-digit
+                                 PIN when a client starts the ceremony)
 
 M0 OPTIONS:
     --source <synthetic|portal|kwin-virtual>