fix(media): stop permanent voice loss after bad-network blip (protocol v2)

A bad UDP packet on a flaky link could permanently wedge the voice path,
unrecoverable even across app restarts. Three defects:

1. Anti-replay window was advanced from the UNAUTHENTICATED header seq
   before the AEAD tag was checked, and not rolled back on failure. One
   corrupted/forged frame shoved recv_highest_ far ahead, after which every
   legitimate frame was rejected as "too old" forever. Reorder to
   replay-check -> authenticate -> update (RFC 3711 3.3); the window now
   moves only after a successful tag check.

2. The wire seq was only the low 16 bits of the nonce counter (zero-extended
   on receive). After 65,536 frames the nonce desynced and all frames failed
   auth. Widen the voice frame seq u16 -> u64 (header 14 -> 20 bytes). The
   core owns all UDP framing, so Swift/C# clients need only a rebuild. This
   is a versioned wire change: VOICECAT_PROTOCOL_VERSION 1 -> 2, handshake
   rejects on mismatch.

3. Server leaked per-session UDP state on disconnect; unregister_session now
   frees udp_endpoints_/udp_tokens_/ssrc_to_session_.

Also add rate-limited dropped-frame logging to MediaRelay so a wedged media
path is observable. New regression tests in test_media_aead.cpp cover the
poison (fails on old code) and the 16-bit wrap. ctest --preset dev
-E external_pcm: 22/22 pass (external_pcm aborts on a pre-existing CoreAudio
shutdown race, unrelated).

docs/protocol.md

@@ -153,6 +153,9 @@ Notes:
 - **Version negotiation.** Each side sends `proto_version` (integer) and a `features`
   string list. The effective version is `min(client, server)`; the effective feature set
   is the intersection. A client that doesn't understand a feature simply never uses it.
+  The **current `proto_version` is 2**. v2 widened the UDP voice frame `seq` field from
+  u16 to u64 (voice.md §2) — a wire-format change with no backward compatibility on the
+  media path, so the server rejects any peer not on v2 rather than min-negotiating down.
 - **Auth over TLS.** Passwords cross the wire only inside TLS 1.3 and are verified against
   an Argon2id hash at rest (see security.md). `auth_methods` in `ServerHello` advertises
   whether `guest` is enabled.