fix(punktfunk/1): adversarial-review fixes — SPAKE2 pairing, renegotiation hardening, +more

Triaged the multi-agent review of the renegotiation + pairing + Sway + AV1/surround batch
(1 critical, 11 major/minor confirmed). Fixes:

CRITICAL — PIN pairing was offline-brute-forceable. The HMAC-of-PIN proof let an active
MITM who terminates the TOFU ceremony recover the 4-digit PIN by offline dictionary search
(all other inputs observable) and forge a correctly-bound proof. Replaced with **SPAKE2**
(balanced PAKE, `spake2` crate) + key-confirmation MACs, binding both cert fingerprints as
the SPAKE2 identities: an attacker gets exactly ONE online guess, no offline search, and
mismatched cert views (a real MITM) never reach a shared key. Also reworked the UX to an
"arming PIN" — one PIN per arming window shown at host startup (the SPAKE2 client needs the
PIN to build its first message, so it can't be minted per-connection). Validated live:
wrong PIN rejected in 0.1s, right PIN pairs + persists + the paired identity streams.

Pairing hardening: `--allow-pairing`/`--require-pairing` must arm pairing (default rejects
unsolicited ceremonies); per-host cooldown bounds online guessing; the client flushes its
CONNECTION_CLOSE so a refused ceremony can't wedge the sequential host for the full timeout;
atomic (temp+rename) paired-store writes.

Protocol: control/pairing messages use a distinct CTL_MAGIC (PKFc) — fully disjoint from
the positional Hello namespace (a future abi_version can't be misparsed as a control
message); all typed decodes are length-exact. ABI_VERSION → 2 (punktfunk_connect signature
gained the identity params; header regenerated).

Renegotiation: drain the reconfig channel to the NEWEST mode (one rebuild, not one per
stale step); validate refresh_hz; build the new pipeline BEFORE dropping the old so a
rebuild failure keeps the session on its current mode instead of killing it.

GameStream: packetDuration snaps to {5,10} (an in-between value isn't a legal Opus frame
size and would kill audio). Sway: chooser file moved to $XDG_RUNTIME_DIR (was a fixed
world-writable /tmp path — DoS / capture-misdirection by another local user).

Swift: fixed two compile breakers in the new pairing/identity APIs (Int32 status .rawValue,
UInt cap cast). New SPAKE2 + namespace-disjointness + pairing-roundtrip unit tests; the
in-process pairing test now also exercises the arming PIN + cooldown. 114 tests green,
clippy -D warnings clean (both feature sets), fmt, C-ABI harness.

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

CLAUDE.md

@@ -38,8 +38,9 @@ Low-latency desktop/game streaming stack, Linux-first, with a shared Rust protoc
   audio** 0xC9 (48 kHz stereo, 5 ms, host→client), **rumble** 0xCA (host→client). **Trust:**
   host serves its persistent identity (`~/.config/punktfunk/cert.pem`, shared with GameStream
   pairing) and logs the SHA-256 fingerprint; clients pin it (TOFU on first connect —
-  `endpoint::client_pinned`), and a **PIN pairing ceremony** (host displays a 4-digit PIN,
-  proof = HMAC over both cert fingerprints, single attempt) establishes mutual trust:
+  `endpoint::client_pinned`), and a **SPAKE2 PIN pairing ceremony** (host arms pairing and displays a
+  4-digit PIN; a PAKE binds both cert fingerprints so an attacker gets one online guess,
+  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`.
   **Mid-stream mode renegotiation**: `Reconfigure` on the still-open control stream — the