Decode CELT frame side information

[zshang-oai]

Summary

This is the last piece before we get to "real meat" :) it's basically table 56 first eight columns.

Implements the first CELT frame side-info slice for RFC 6716 Section 4.3 work:

• add CELT frame configuration validation and side-info parsing through the intra-energy flag
• decode silence, pitch post-filter header, transient, and intra-energy flags in RFC Table 56 order
• add rangecoding.Decoder.DecodeUniform for the RFC 6716 Section 4.1.5 ec_dec_uint() path needed by the post-filter octave
• add a CELT-side range trace test harness that records Tell, TellFrac, remaining bits, and final range at named decode milestones

[codecov]

Codecov Report

❌ Patch coverage is 93.25843% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.56%. Comparing base (085f78c) to head (9d9c788).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/celt/frame.go	90.76%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
+ Coverage   80.98%   81.56%   +0.58%     
==========================================
  Files          15       16       +1     
  Lines        2135     2224      +89     
==========================================
+ Hits         1729     1814      +85     
- Misses        341      343       +2     
- Partials       65       67       +2     

Flag	Coverage Δ
go	81.56% <93.25%> (+0.58%)	⬆️

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[Copilot]

Pull request overview

Implements the initial CELT frame “side information” decoding slice (per RFC 6716 §4.3 / Table 56 order) and adds supporting range-coder functionality plus trace-based tests to validate range decoder state at key milestones.

Changes:

• Add rangecoding.Decoder.DecodeUniform() to support RFC 6716 ec_dec_uint() decoding (range-coded prefix + raw-bit suffix).
• Introduce CELT frame config validation and side-info parsing through the intra-energy flag (silence, post-filter header, transient, intra-energy).
• Add CELT range-trace test harness and targeted unit tests for the new decoding path.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/rangecoding/decoder.go	Adds uniform integer decoding (DecodeUniform) needed for CELT post-filter parsing.
internal/rangecoding/decoder_test.go	Adds unit tests covering DecodeUniform behavior (in-range, suffix combine, saturation, degenerate totals).
internal/celt/frame.go	Implements CELT side-info parsing and frame config validation.
internal/celt/frame_test.go	Adds tests for config validation, silence handling, post-filter decode, transient/intra flags, and range trace checkpoints.
internal/celt/trace_test.go	Adds a reusable trace helper for asserting range-decoder state at named checkpoints.
internal/celt/errors.go	Adds CELT-specific errors for invalid channel counts and range coder symbol issues.

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[Copilot]

decodeSilenceFlag only decodes the silence symbol when tell == 1. That’s currently true immediately after rangeDecoder.Init(), but if the call site changes (or the decoder is reused without re-init) the function will silently skip decoding even when tell < totalBits, leaving silence at its zero value. Consider removing the tell == 1 guard (decode whenever tell < totalBits) or documenting/enforcing the precondition that this function must be called right after Init().

Suggested change

	case tell == 1:
	default: