docs: architecture describes what is built now

kernel (threads, demand paging, TLB shootdowns, dynamic linking, RAM direct map), syscalls, filesystem (/proc, /sys,
/dev), terminals (12, VT handshake, ptys), networking (r8169 through LDLS), LDLS (module sets; Linux code may ship),
devices (what stays in the kernel; hybrid.md's lasting content), boot (the loader's placement of the kernel),
graphics (fbdev, GMMOS, QR 1-40), audio (ALSA over OSS), debugging, usb. New: packages.md (the apk root) and
desktop.md (Xorg on the framebuffer).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EVRhxi9zpdW2uDUN4FVcao

docs/architecture/audio.md

@@ -40,7 +40,7 @@ until the speaker has caught up*.
 **It is one stream.** Mixing several programs into one device is a real feature
 with a real design behind it, and building it before anything had played a
 single sound would be exactly the mistake [../plan/1.0.md](../plan/1.0.md)
-keeps a register for: work that exists and is connected to nothing. The second
+keeps a register against: work that exists and is connected to nothing. The second
 program that wants to play at the same time is what should cause it to be
 written.
 
@@ -84,6 +84,25 @@ Both register files are **I/O ports**, not MMIO — which is why nothing in the
 driver maps anything. `bar[n].is_io` is what says so, and a driver that
 `ioremap`ed these would be mapping a port number as though it were an address.
 
+### Through ALSA, for programs that expect it
+
+SDL 3 has no OSS backend and SchOS has no ALSA *kernel* side, so sound from SDL games
+and anything else that speaks ALSA goes **SDL → alsa-lib → alsa-plugins' OSS plugin →
+`/dev/dsp`**. `alsa-lib` and `alsa-plugins` are in the Alpine `world`, and
+`/etc/asound.conf` (from `schos-base`) makes the default device a `plug` onto the OSS
+device at 16-bit stereo 48 kHz, which is what every SchOS audio driver plays.
+
+Running that path found two bugs underneath it:
+
+- **AC'97:** resetting the PCM-out box also clears the descriptor-list address and nothing
+  set it again, so 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` was missing the ioctls the plugin's ring-and-pointer model uses:**
+  `SETFRAGMENT`, `GETFMTS`, `GETCAPS`, `SETTRIGGER`, `GETOPTR`, `GETODELAY`, 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.
+
 ## How it is tested
 
 QEMU's `wav` audio backend writes what the guest plays to a file, so the test