sceAudio: implement sceAudioOneshotOutput, and cover the rest of the channel rules

Another pass looking for gaps, all of it now recorded by audio/blocking/channels
and audio/blocking/oneshot.

sceAudioOneshotOutput was the last unimplemented entry in the module, returning
"library not linked" to anyone who called it. It plays one buffer on a channel it
never reserves, so the channel frees itself when the buffer runs out, and its
argument checks are their own set: any positive sample count, aligned or not, no
upper bound, a negative volume rejected rather than skipped, and no busy check at
all. No game is known to use it; it is implemented because tracing it turned out
to be cheap, not because anything needed it.

The channels test confirms four behaviors: sceAudioChReserve(-1) skips a released
channel that is still playing while a reserve of it succeeds, a second channel
joining a running mixer doesn't lose a block unlike a first, mono counts down in
the same 64-sample steps over the same time as stereo, and the panned blocking
output has no extra delay on the high channels.

Also: the SRC resampler now interpolates into the next buffer at a buffer join
instead of holding the last sample, since the codec reads the two descriptors as
one stream. That only shows up at non-native rates and no test can see it.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

docs/sceAudio.md

@@ -150,6 +150,18 @@ checks is observable.
 Accepted SRC frequencies are 8000, 11025, 12000, 16000, 22050, 24000, 32000 and 48000, plus
 whatever the output is currently running at - which is how 44100 and 0 get through.
 
+## sceAudioOneshotOutput
+
+The odd one out: it plays a single buffer on a channel it never reserves. The channel stays
+free as far as everything else is concerned, so `sceAudioChRelease` on it answers `80260008`,
+and it returns to the pool by itself once the buffer runs out. It returns the channel number,
+takes any positive sample count - alignment and the 0xFFC0 ceiling do not apply - and unlike
+the other outputs a negative volume is an error rather than "leave it alone". There is no busy
+check either: a second one-shot while the first is playing just replaces it.
+
+No game is known to call it. It is implemented because the behavior turned out to be simple
+once traced, not because anything needed it.
+
 ## sceVaudio is a third shape
 
 `sceVaudio` reserves the same channel as Output2 and SRC, but its own release is not the same