mod_sndfile: allow overriding the WAV write subtype via a file-string param

[celliso1]

Description

On write, mod_sndfile unconditionally OR-s SF_FORMAT_PCM_16 into the format
for every standard sample rate, and no file extension maps a WAV container to a
companded (u-law/A-law) payload — .ul/.ulaw produce a headerless
SF_FORMAT_RAW file, not RIFF/WAVE. As a result there is no way to record, for
example, a G.711 call into a RIFF/WAVE file whose payload stays u-law: the
recording is always expanded to 16-bit PCM, doubling the size on disk and
losing the bit-exact relationship to what crossed the wire.

This adds an optional subtype parameter to the standard file-string prefix,
so the caller can choose the encoded subtype while the container stays implied
by the extension:

session:recordFile("{subtype=ulaw}/tmp/foo.wav")

Accepted values: ulaw, alaw, pcm16, pcm24, pcm32. Write mode only —
reads continue to autodetect the subtype from the header. If the requested
subtype is not valid for the container implied by the extension (for example
ulaw on a .flac target), mod_sndfile logs a warning and falls back to the
container's default subtype rather than failing the open; an unrecognized value
is likewise ignored with a warning. The change is therefore inert unless a
caller explicitly requests a supported subtype, and a bad param never costs the
recording.

Recording a G.711 call as u-law WAV is lossless with respect to the wire, half
the size of the PCM-16 file produced today, and trivially decodable by anything
that reads WAV.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Code cleanup / refactor

Related Issues

No existing issue found for companded WAV write support; happy to open a
tracking issue if the maintainers prefer one.

Testing

• Added/updated unit tests
• Tested manually
• Tested with live SignalWire credentials (if applicable)

Unit tests were added to tests/unit/switch_core_file.c:

• {subtype=ulaw} on a .wav target produces a RIFF/WAVE file with format
  tag 7 (ITU G.711 u-law) and 8-bit samples that decodes back within u-law
  quantization tolerance.
• An unrecognized subtype falls back to PCM_16 (format tag 1).
• An unsupported container/subtype pair ({subtype=ulaw} on a .flac target)
  still opens successfully and yields a valid FLAC file — proving the
  warn-and-fall-back path rather than a failed open.

Manual: recorded a live inbound G.711u call via record_session to a
{subtype=ulaw} .wav target, confirmed the container/subtype with file(1)
(RIFF/WAVE, format tag 7, 8-bit), then decoded and played the file back and
re-transcribed it successfully.

Checklist

• I have read the CONTRIBUTING guidelines
• My code follows the project's style guidelines
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)
• All existing tests pass

Additional Notes

Why not just re-encode to a compressed format?
For telephony audio that will be archived and later transcribed or analyzed,
re-encoding the recording to a low-bitrate lossy codec (e.g. MP3) is a poor
fit: lossy compression at telephony bitrates degrades the exact tokens that
downstream ASR/NLP extract — digit sequences and spelled names — even when
overall intelligibility is preserved. This is not hypothetical: internal A/B
testing that transcribed lossy-re-encoded copies against losslessly-stored
copies of the same calls showed a measurable, repeatable increase in misspelled
names and corrupted number sequences. Storing the native companded payload in a
WAV container sidesteps that entirely — lossless with respect to the wire, half
the size of PCM-16, and readable by any WAV consumer.

Documentation

The parse site in mod_sndfile.c carries an inline comment
describing the parameter and its accepted values. The wiki/params entry below is
ready to drop into the mod_sndfile / file-string documentation so the parameter
is discoverable outside the source:

subtype (write)

Overrides the encoded sample format (subtype) that mod_sndfile writes,
while the container is still selected by the file extension. Supplied as a
file-string prefix param, e.g. via record_session, recordFile, or any
API that opens a file through mod_sndfile:

record_session {subtype=ulaw}/tmp/foo.wav      # RIFF/WAVE container, u-law payload
uuid_record <uuid> start {subtype=alaw}/tmp/foo.wav
session:recordFile("{subtype=pcm24}/tmp/foo.wav")

Value	Encoded subtype	Notes
ulaw	ITU G.711 µ-law, 8-bit	half the size of PCM-16, lossless vs. a G.711µ leg
alaw	ITU G.711 A-law, 8-bit	half the size of PCM-16, lossless vs. a G.711a leg
pcm16	16-bit signed PCM	current default
pcm24	24-bit signed PCM
pcm32	32-bit signed PCM

• Write only. On read, mod_sndfile autodetects the subtype from the
  file header; the param is ignored.
• An unrecognized value is ignored (a warning is logged) and the
  container's default subtype is kept.
• A subtype the container cannot hold (for example ulaw on a .flac
  target) is validated with libsndfile's sf_format_check(); if it fails,
  mod_sndfile logs a warning and falls back to the container's default
  subtype rather than failing the open. A bad subtype never costs the
  recording — worst case you get the container's default encoding.

[celliso1]

cc @andywolk — small, self-contained follow-up in the same area as #3061 (compute/storage cost). This adds an opt-in file-string param to override the WAV write subtype in mod_sndfile; default behavior is unchanged. Would appreciate a review when you have a moment. @jakubkarolczyk @s3rj1k in case you're closer to the formats code.