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>

crates/punktfunk-host/src/main.rs

@@ -91,6 +91,7 @@ fn real_main() -> Result<()> {
                     .and_then(|s| s.parse().ok())
                     .unwrap_or(0),
                 require_pairing: args.iter().any(|a| a == "--require-pairing"),
+                allow_pairing: args.iter().any(|a| a == "--allow-pairing"),
                 pairing_pin: None,
                 paired_store: None,
             })
@@ -320,8 +321,9 @@ 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)
+    --allow-pairing              accept PIN pairing ceremonies (arm pairing mode)
+    --require-pairing            only serve PIN-paired clients (implies --allow-pairing;
+                                 the host logs a 4-digit PIN when a client starts pairing)
 
 M0 OPTIONS:
     --source <synthetic|portal|kwin-virtual>