Use the midpoint for unsigned 8-bit playback silence

[antonsoo]

The convenience playback callback fills unmapped channels and the final partial block with zero. For uint8, zero is the negative endpoint; silence is 128, as documented by SoundDevice and PortAudio. This can send a DC signal on a channel intended to be silent.

Choose the fill value from the output dtype and use it at both sites.

Validation:

• Native PortAudio/ALSA file playback of two WAV assets repairs 14 of 48 configurations and 12,498 incorrect silence samples. All audio payloads and 34 complete callback-window controls stay exact. These checks use isolated file/null PCMs.
• The existing CI import/device/version smoke commands and Pyright source/example checks pass locally. Hardware and other operating systems were not tested.

[mgeier]

Thanks for this PR!

Similar to #644, this adds both a new feature and a new testing framework. Those should be two separate discussions.

[antonsoo]

Thanks. I removed the standalone unittest file in 87708d0. The PR now changes only the two silence fills; testing infrastructure can be discussed separately.

[mgeier]

Thanks!