Add stereo mid/side encoding

[thomas-vilte]

Description

Adds stereo encoding to the CELT layer. WithChannels(2) now produces valid stereo packets end-to-end: the public API deinterleaves the input, analysis runs per-channel, and quantAllBandsStereo handles mid/side coupling — mirroring the decoder's existing quantBand stereo path.

The theta encoder implements the same three probability models as decodeBandTheta (stereo-large with p0=3 at extremes, uniform, triangular). The stereo path uses midGain=1, level=0 for recursive calls, and nil lowband for the side channel — matching what the decoder expects. A FinalRange comparison test between encoder and decoder catches any range coder divergence after a stereo frame.

Intensity stereo and the dual-stereo L1 decision are left for follow-up PRs. For now, intensity=maxBands (disabled) and mid/side coupling is always used.

Reference issue

Fixes #...

[codecov]

Codecov Report

❌ Patch coverage is 76.08696% with 77 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.77%. Comparing base (7dc8324) to head (ac16983).

Files with missing lines	Patch %	Lines
internal/celt/encode_bands.go	73.48%	58 Missing and 12 partials ⚠️
internal/celt/encoder.go	76.47%	2 Missing and 2 partials ⚠️
internal/celt/analysis.go	92.30%	1 Missing and 1 partial ⚠️
encoder.go	93.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #126      +/-   ##
==========================================
+ Coverage   82.88%   83.77%   +0.89%     
==========================================
  Files          29       29              
  Lines        6110     6398     +288     
==========================================
+ Hits         5064     5360     +296     
+ Misses        811      801      -10     
- Partials      235      237       +2     

Flag	Coverage Δ
go	83.77% <76.08%> (+0.89%)	⬆️

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