audio/pipewire-spa-oss-ng: New port

PipeWire SPA plugin implementing a FreeBSD OSS backend, tracking the
pw-oss fork at https://github.com/kev009/pw-oss.

Changes over upstream shkhln/pw-oss:

Audio / OSS:

• Size the OSS buffer to the device and fix data-loop aborts.
• Adapt the fill target to the device's real fragment size.
• Report clock delay and rate; require MemPtr buffers.
• Reset the channel before close so close() doesn't block draining.
• Close the device when the format is cleared.
• Advertise and re-emit the port Format param (ALSA-sink pattern).
• Don't abort the process on an oversized chunk or a bad/unpositioned format.

Session / config:

• Don't launch a second WirePlumber from exec.conf; the stock config already starts one, and the duplicate bricked audio.

Packaging / dependencies:

• Refresh all dependencies; migrate to libspa 0.10 (its pod builder now returns rustix::io::Errno).
• Rename the crate to spa-freebsd-oss-ng; the installed libspa_freebsd_oss.so is unchanged.
• Add package metadata and install docs for the audio/pipewire-spa-oss-ng port.
• Quiet clippy and dead_code warnings.

Conflicts with audio/pipewire-spa-oss as both install the same files.