fix(descriptor): rebuild SDP that Firefox accepts; release v5.8.1

Live cross-browser testing found three defects in the SBQ2 SDP template, all
invisible to Chrome and all fatal to Firefox.

The candidate lines omitted raddr/rport on srflx and relay candidates. RFC 8839
section 5.1 makes rel-addr and rel-port mandatory for non-host candidates even
though ICE never reads them; Chrome tolerates the omission and Firefox drops the
candidate. Relay-only connections to Firefox failed 0/8 against 8/8 for the
browser's own SDP. The STUN and TURN profiles hid it because a host pair
connected instead -- the relay candidates were never actually needed there.

The template also advertised ice-options:trickle without ever closing the
candidate set, though a descriptor is a complete one-shot set with no channel to
trickle over, and hard-coded the m= port and c= line to the 9 / 0.0.0.0 null
default candidate, which is the trickle convention for "nothing gathered yet".
Both are now correct: no trickle, an explicit a=end-of-candidates, and the most
publicly reachable candidate as the default.

All three are serializer-side and cost zero descriptor bytes; sizes are
unchanged at 98-149 bytes, QR version 6-8. Verified 48/48 across all 16
combinations of {Chrome, Firefox} squared and four network profiles, with every
relay-only pair now connecting over the relay.

README.md

@@ -9,7 +9,7 @@
 No accounts. No servers storing your messages. No installation required.
 
 [![License: MIT](https://img.shields.io/badge/License-MIT-f0892a.svg)](LICENSE)
-[![Version](https://img.shields.io/badge/version-5.8.0-3ecf8e.svg)](CHANGELOG.md)
+[![Version](https://img.shields.io/badge/version-5.8.1-3ecf8e.svg)](CHANGELOG.md)
 [![PWA](https://img.shields.io/badge/PWA-installable-3ecf8e.svg)](#install-as-an-app)
 [![Encryption](https://img.shields.io/badge/crypto-ECDH%20P--384%20%C2%B7%20AES--256--GCM-blue.svg)](#security-model)
 [![Forward secrecy](https://img.shields.io/badge/forward%20secrecy-Double%20Ratchet-3ecf8e.svg)](#forward-secrecy)