v5.18

test: disable real audio in the headless test harness

The release gate (python3 -m tests.run --all) hung indefinitely in the gui
sound tests: a looping ambient/search bed kept re-decoding via Qt-multimedia/
ffmpeg, flooding stderr with "mp3float: Could not update timestamps" and
never converging. The harness forced QT_QPA_PLATFORM=offscreen but left real
audio enabled, so headless runs drove actual audio hardware - flaky across
interpreters/CI runners (it has also segfaulted on teardown).

Default SSLAX_NO_AUDIO=1 alongside the offscreen platform. The sound engine
logic is fully covered via the stubbed engine + monkeypatching, so no
coverage is lost; the suite stays green (1035 passed, 0 failed) and is now
deterministic for the local gate and CI. Opt back in with SSLAX_NO_AUDIO="".

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>