v0.5.0

SIP conformance overhaul and a major feature batch: this release makes
aiosipua hold its own against real-world SIP servers and adds the
signaling features a voice-AI backend needs.

Breaking changes

• SipUAC.send_cancel(call) replaces send_cancel(dialog, remote_addr) —
  the CANCEL is built from the original INVITE (same branch and CSeq,
  RFC 3261 §9.1), which is what proxies require to match it.
• SipMessage.body is now bytes (SIP bodies are octet streams). Read or
  write text bodies through the new message.text property; parse_sdp
  raises a TypeError pointing at .text if handed bytes.

Conformance fixes

• Digest authentication rewritten per RFC 7616: qop="auth" (cnonce/nc),
  SHA-256 (RFC 8760) alongside MD5, opaque echoed — challenges from
  Asterisk/FreeSWITCH/Kamailio now succeed.
• re-INVITE 200s are ACKed with their own CSeq; on_answer no longer replays.
• In-dialog requests are validated against the dialog (wrong tags → 481,
  non-increasing CSeq → 500); CANCEL matches the INVITE transaction by branch.
• SDP answers mirror every offered m-line in order, port 0 for rejected
  streams (RFC 3264 §6).
• received/rport stamped on incoming Vias (RFC 3581); advertised_addr
  on UAC/UAS for NATed signaling.

New features

• Reliability: 200 OK retransmitted until ACK over UDP, reliable
  provisionals with PRACK/100rel (RFC 3262), automatic transaction expiry.
• REGISTER client with auto-refresh, 423 handling, and expiry watchdog.
• UPDATE (RFC 3311) and blind transfer via REFER (RFC 3515) with
  transfer-progress NOTIFYs.
• Session timers (RFC 4028): UPDATE refreshes and dead-call watchdogs.
• IPv6 end to end: bracketed literals in Via/Contact/URIs, IN IP6 SDP.

Hardening

• Header-injection guards on all header setters; size caps on header count,
  TCP header bytes, and declared body length; required-header validation.
• Parser survives the RFC 4475 torture suite and holds two hypothesis-checked
  invariants: only ValueError on arbitrary input, and a stable wire format
  after one normalization pass.

344 → 524 tests; every module under strict mypy.