Audio: silent GetBuffer, 7.1.4 fold-down, ring-fill pacing

[spitefulowl]

Why

Ori and the Will of the Wisps played nothing but crackle. Its Wwise renders through Wine's ISpatialAudioObjectRenderStream, and two problems added up.

Commits

1. Hand out silence from GetBuffer and fold a 7.1.4 bed to stereo (ml1227).
   • spatialaudio.c mixes each static object into the render buffer with *out += *in but never clears it. WASAPI leaves GetBuffer contents undefined, and ios_get_render_buffer handed back the same scratch area every period, so every period was added onto the previous one. The source level of the 12-channel stream climbed from mean |x| 0.01 to 5. winecoreaudio and winepulse do not clear the buffer either, so upstream Wine has the same latent bug.
   • ios_get_render_buffer now zeroes the frames it hands out.
   • The stereo fold-down now honours channel masks up to 12 channels. The top speakers go to their own side at -3 dB and the top centres to both sides. Before, channels 9-12 were dropped.
2. Wake the event client by ring fill, not by the clock (ml1230).
   • The game delivered 212 s of audio in ~230 s of gameplay: a 10 ms hole every ~120 ms. The spatial stream asks for exactly one period per event, and the timer thread signalled after every usleep(10000), so every late wake-up was 10 ms of audio never written.
   • The timer loop now keeps a lead of MADEIRA_AUDIO_LEAD_MS queued (default 60 ms, 10-500). Below the lead it signals again as soon as the client has written; at or above the lead it sleeps until the lead would be used up.
   • The timer thread runs under THREAD_TIME_CONSTRAINT_POLICY, as Core Audio's I/O thread does.
   • The RT callback counts short passes and clamped samples, and a [ios_audio] ml1230 pacing line is printed every ~10 s per live stream.
   • Null mode keeps the plain 10 ms beat.

Testing

• ml1227, on the device (Ori and the Will of the Wisps, 2026-10-02): the source level stayed bounded (peak <= 0.63, no clipping) and the sound became recognisable. check-audio-downmix.py gains a 7.1.4 case, which fails before this change and passes after.
• ml1230, on the device: clean sound by ear. 38 of 39 ten-second windows wrote 99.8-100.4% with 0 short passes. The one bad window was a 37 s load at 4-7 FPS, during which the game's own audio thread did not run.
• ml1230 trade-off: latency grows to ~60 ms in the ring plus the RemoteIO buffer.
• Load-order fix, built only: a fix in the pacing loop, made after the device build, is only syntax-checked. The loop now loads play_pos before write_pos and clamps the padding at 0.

Notes

No binary is included. libntdll_unix.a is built by build/ntdll-unix/build.sh.

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