Sound through ALSA, a 2D game, and a stack walk that cannot panic

SDL 3 has no OSS backend and SchOS has no ALSA kernel side, so sound now
goes SDL -> alsa-lib -> alsa-plugins' OSS plugin -> /dev/dsp. That path
found two bugs underneath it:

- ac97: resetting the PCM-out box also clears the descriptor list address,
  and nothing set it again. Every stream after the first stop played a
  "list" at physical address 0 -- silence -- and ALSA resets before each
  stream. If the engine halts at the end of the queue, the run bit is set
  again.
- /dev/dsp: SETFRAGMENT, GETFMTS, GETCAPS, SETTRIGGER, GETOPTR and GETODELAY
  for the plugin's ring-and-pointer model, and poll(), writable once a whole
  fragment fits (ALSA spun at 76% CPU otherwise). speaker-test now plays in
  real time at 0% CPU, verified in a recording of QEMU's AC'97.
- /etc/asound.conf: the default device is a plug to the OSS device at 16-bit
  stereo 48 kHz, what every SchOS audio driver plays.

Also:

- getstackpcs: each frame link must go up the same stack by less than
  KSTACKSIZE. The chain ends at the syscall entry with the user's rbp,
  which lbreakouthd uses as a register (-1024); the walk dereferenced it
  under the syscall tracer and the kernel panicked inside acquire().
- fb: FBIOGET_VSCREENINFO reports nominal timings, as Linux's efifb does,
  instead of zeros, which Xorg's fbdev turned into a 0 Hz RandR mode.
- procdump (Ctrl-P) shows where runnable threads are in user space.
- World: lbreakouthd instead of lbreakout2 (its stk.c passes vsnprintf a
  size of 511 plus the prefix length, and Alpine's fortified headers trap on
  that), plus curl, alsa-lib and alsa-plugins.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

tools/posix/mkaroot.sh

@@ -211,6 +211,32 @@ exec "$@"
 BEOF
 chmod 755 "$P/usr/bin/bwrap"
 
+# ALSA's default device, through the OSS plugin to /dev/dsp. The plug in front
+# converts whatever a program plays to what every SchOS audio driver accepts:
+# 16-bit stereo at 48 kHz (AC'97 without variable rate plays nothing else, and
+# the plugin refuses a rate the device rounds by more than 1%).
+mkdir -p "$P/etc"
+cat > "$P/etc/asound.conf" <<'AEOF'
+# SchOS: ALSA plays through OSS's /dev/dsp (kernel/dev/audio.c).
+pcm.!default {
+    type plug
+    slave {
+        pcm "dsp"
+        format S16_LE
+        channels 2
+        rate 48000
+    }
+}
+pcm.dsp {
+    type oss
+    device /dev/dsp
+}
+ctl.!default {
+    type hw
+    card 0
+}
+AEOF
+
 # Mount points and runtime directories are alpine-baselayout's; an empty
 # directory here only duplicates it.
 for d in dev proc sys run tmp srv; do