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>

Author: leonxger

tests/harness.py

@@ -1,9 +1,9 @@
 """Test harness: headless bootstrap, save discovery, the result model and the
 shared Context (lazy QApplication + cached MainWindow + per-save decode verdicts).
 
-Importing this module forces ``QT_QPA_PLATFORM=offscreen`` BEFORE any Qt import,
-so every section is headless-safe. Sections never construct a QApplication or load
-reference data themselves; they ask the Context for what they need.
+Importing this module forces ``QT_QPA_PLATFORM=offscreen`` and ``SSLAX_NO_AUDIO=1``
+BEFORE any Qt import, so every section is headless-safe. Sections never construct a
+QApplication or load reference data themselves; they ask the Context for what they need.
 """
 
 import os
@@ -14,6 +14,13 @@
 
 # Headless before anything imports PySide6. Must run at import time.
 os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
+# Never drive REAL audio hardware from the test suite. Qt-multimedia/ffmpeg playback in
+# an offscreen/headless context is flaky across interpreters and CI runners - it has both
+# hung (a looping bed re-decoding forever, stalling the release gate) and segfaulted on
+# teardown. The sound ENGINE logic is covered via the stubbed engine + monkeypatching, so
+# disabling real playback loses no coverage while making the suite deterministic everywhere
+# (local gate AND CI). Opt back into real audio with SSLAX_NO_AUDIO="" if ever needed.
+os.environ.setdefault("SSLAX_NO_AUDIO", "1")
 
 REPO_ROOT = Path(__file__).resolve().parent.parent
 if str(REPO_ROOT) not in sys.path: