Add public encoder API

[thomas-vilte]

Description

Adds the public Opus encoder API as a wrapper around the internal CELT encoder.

Exposes NewEncoder, Encode (S16LE), EncodeFloat32, SetBitrate, and
SetComplexity at the package root, mirroring the existing Decoder API.

Current scope: 48 kHz, mono, 20 ms CELT-only frames. Stereo, transient
detection, and SILK encoding will land in follow-up PRs.

Round-trip tested against the existing decoder.

Reference issue

Fixes #34

[codecov]

Codecov Report

❌ Patch coverage is 83.13253% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.66%. Comparing base (1b1e053) to head (eb6cb11).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
encoder.go	82.71%	7 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #117      +/-   ##
==========================================
- Coverage   82.88%   82.66%   -0.23%     
==========================================
  Files          26       28       +2     
  Lines        5540     6046     +506     
==========================================
+ Hits         4592     4998     +406     
- Misses        726      812      +86     
- Partials      222      236      +14     

Flag	Coverage Δ
go	82.66% <83.13%> (-0.23%)	⬇️

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[Copilot]

Pull request overview

This PR adds a first public Opus encoder API at the package root, implemented as a thin wrapper around the internal CELT encoder, to complement the existing public decoder API and address #34 (PCM → Opus encoding support).

Changes:

• Introduces Encoder with NewEncoder, Init, Encode (S16LE), and EncodeFloat32 for 48 kHz mono 20 ms CELT-only packets.
• Adds encoder controls SetBitrate and SetComplexity plus new internal sentinel errors for validation.
• Adds round-trip tests that encode then decode back using the existing decoder.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
errors.go	Adds new encoder-related sentinel errors (bitrate/complexity/input/frame validation).
encoder.go	Implements the public encoder wrapper API around the internal CELT encoder and constructs a valid Opus packet (TOC + one frame).
encoder_test.go	Adds round-trip and validation tests for the new encoder API.

Comments suppressed due to low confidence (1)

encoder.go:90

• Doc comment says "CELT encoder slice"; this appears to be a typo/incorrect term (likely meant "encoder implementation" or "API surface"). Consider adjusting wording to avoid confusion.

// SetComplexity stores the requested encoder complexity.
//
// The initial CELT encoder slice does not yet vary behavior by complexity,
// but the public API accepts the standard Opus 0..10 range for future
// expansion.
func (e *Encoder) SetComplexity(complexity int) error {

---

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[JoTurk]

@thomas-vilte @FrantaBOT Should have squash merged or rebased before merge as we usually do a single commit per PR, unless they are standalone commits, if we merge multiple commits in a single PR.

[FrantaBOT]

@thomas-vilte @FrantaBOT Should have squash merged or rebased before merge as we usually do a single commit per PR, unless they are standalone commits, if we merge multiple commits in a single PR.

😓 Sorry, clicked the wrong button

[JoTurk]

npnp.