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>

clients/apple/Sources/PunktfunkKit/PunktfunkConnection.swift

@@ -71,9 +71,9 @@ public struct ClientIdentity: Sendable {
 public func generateIdentity() throws -> ClientIdentity {
     var cert = [CChar](repeating: 0, count: 4096)
     var key = [CChar](repeating: 0, count: 4096)
-    let rc = punktfunk_generate_identity(&cert, cert.count, &key, key.count)
-    guard rc.rawValue == PUNKTFUNK_STATUS_OK.rawValue else {
-        throw PunktfunkClientError.status(rc.rawValue)
+    let rc = punktfunk_generate_identity(&cert, UInt(cert.count), &key, UInt(key.count))
+    guard rc == PUNKTFUNK_STATUS_OK.rawValue else {
+        throw PunktfunkClientError.status(rc)
     }
     return ClientIdentity(certPEM: String(cString: cert), keyPEM: String(cString: key))
 }
@@ -88,6 +88,9 @@ public func pair(
     timeoutMs: UInt32 = 90_000
 ) throws -> Data {
     var observed = [UInt8](repeating: 0, count: 32)
+    // The C header types PunktfunkStatus as a bare int32 (C17, no enum import), so the ABI
+    // functions return Int32 directly — compare against the enum constants' rawValue, the
+    // same bridging the connection methods use (statusOK etc.).
     let rc = host.withCString { cs in
         identity.certPEM.withCString { cert in
             identity.keyPEM.withCString { key in
@@ -99,10 +102,10 @@ public func pair(
             }
         }
     }
-    switch rc.rawValue {
+    switch rc {
     case PUNKTFUNK_STATUS_OK.rawValue: return Data(observed)
     case PUNKTFUNK_STATUS_CRYPTO.rawValue: throw PunktfunkClientError.wrongPIN
-    default: throw PunktfunkClientError.status(rc.rawValue)
+    default: throw PunktfunkClientError.status(rc)
     }
 }