native ambisonics support

[janpbuethe]

Native Ambisonics Support
Summary
Adds native multi-channel ambisonics encoding/decoding to OAC, supporting ambisonics orders 0–5 (1, 4, 9, 16, 25, or 36 channels). In ambisonics mode, each channel is encoded independently per band using CELT, bypassing the stereo mid/side coupling path entirely.

Key Changes
New API surface (include/oac.h, include/oac_defines.h)

New format constants: OAC_FORMAT_STANDARD (mono/stereo) and OAC_FORMAT_AMBISONICS (up to 36ch)
oac_encoder_create(), oac_decoder_create(), and their _init/_get_size variants now take a format parameter
New CTL: OAC_GET_FORMAT to query the initialized format
CELT band quantization (celt/bands.c, celt/bands.h)

Renamed the existing oaci_quant_all_bands to quant_all_bands_twoch (static, handles 1–2 channels)
Added quant_all_bands_multi — a multi-mono quantization path where each channel is encoded independently per band with its own spectral folding norm, lowband scratch, and split memory
New public dispatcher oaci_quant_all_bands routes to the appropriate implementation based on channel count
Bit allocation (celt/rate.c)

Replaced hardcoded >>stereo shifts with
C
division to generalize fine-energy bit allocation for arbitrary channel counts
Energy quantization (celt/quant_bands.c)

Expanded prev[] arrays from fixed size 2 to OAC_MAX_CHANNELS for multi-channel coarse energy quantization/unquantization
Encoder/Decoder (src/oac_encoder.c, src/oac_decoder.c, celt/celt_encoder.c, celt/celt_decoder.c)

Encoder and decoder state structs carry a format field
Format/channel validation via oaci_validate_format_channels() in oac_private.h
Ambisonics mode forces CELT-only encoding (no SILK) and OAC_APPLICATION_AUDIO
Multistream (src/oac_multistream_encoder.c, src/oac_multistream_decoder.c)

Updated to pass format through to underlying encoder/decoder instances
Tests

New tests/test_oac_ambisonics.c — encode/decode round-trip tests for all valid ambisonics channel counts
Usage — oac_demo with ambisonics input

Encode a 4-channel (1st order) ambisonics file

./oac_demo -e -format ambix input_4ch.pcm encoded.oac 48000 4 64000

Decode back

./oac_demo -d -format ambix encoded.oac output_4ch.pcm 48000 4

Encode + decode in one pass (default)

./oac_demo -format ambix input_4ch.pcm output_4ch.pcm 48000 4 64000

-format ambix enables ambisonics mode (default is std for standard mono/stereo)

• Valid channel counts: 1 (0th order), 4 (1st), 9 (2nd), 16 (3rd), 25 (4th), 36 (5th)
• Input/output are raw interleaved 16-bit PCM
• Bitrate is the total bitrate across all channels

Note that channel count is an out-of-band parameter for ambix i.e. we do not support order conversion (yet).

[thirv]

Left couple comments/questions.

[trsonic]

Could the support be extended to 7OA?

[janpbuethe]

Could the support be extended to 7OA?

That's relatively easy to do.

[thirv]

As we build the multichannel capacity, there are number of tools that people are interested in having to handle correlations and joint bit allocation. Clarifying the former, it seems that we want to include the mechanism of 1) constructing a downmix and 2) demixing at the decoder. Regardless of the methods of how to actually implement them, either/both of these could be static or dynamic.

I'd advocate to keep these possibilities open, and this PR seems like it would be a valid basis for adding these later. Feel free to discuss if the above does not resonate.