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-core/Cargo.toml

@@ -19,7 +19,7 @@ crate-type = ["lib", "cdylib", "staticlib"]
 default = []
 # Control-plane QUIC (pairing, config, reverse audio). tokio is permitted ONLY here,
 # never on the per-frame hot path. Off by default so the core stays runtime-free.
-quic = ["dep:quinn", "dep:tokio", "dep:rustls", "dep:rcgen", "dep:rustls-pki-types", "dep:sha2", "dep:hmac"]
+quic = ["dep:quinn", "dep:tokio", "dep:rustls", "dep:rcgen", "dep:rustls-pki-types", "dep:sha2", "dep:hmac", "dep:spake2"]
 
 [dependencies]
 reed-solomon-simd = "3.1"      # GF(2^16) Leopard-RS, SIMD, O(n log n) — the wall-breaker (P2)
@@ -42,6 +42,7 @@ rcgen = { version = "0.13", optional = true, default-features = false, features
 rustls-pki-types = { version = "1", optional = true }
 sha2 = { version = "0.10", optional = true }
 hmac = { version = "0.12", optional = true }
+spake2 = { version = "0.4", optional = true }
 tokio = { version = "1", optional = true, features = ["rt-multi-thread", "net", "sync", "macros"] }
 
 [dev-dependencies]