audio: revive the opus package

Operating System

• Windows
• macOS
• Linux
• FreeBSD
• OpenBSD
• Android
• iOS
• Nintendo Switch
• PlayStation 5
• Xbox
• Web Browsers

What feature would you like to be added?

Bring back audio/opus, providing Decode and DecodeF32 for Opus streams alongside audio/vorbis and audio/mp3.

The package was added in #3393 and removed as part of #3621, so the API, its tests, the Opus mode in examples/audio, and the ragtime.opus sample under examples/resources/audio can all be restored from history. What is missing is a decoder that is sound on every supported platform:

• It must work on 32-bit platforms. The previous backend github.com/kazzmir/opus-go is transpiled C generated for linux/amd64 that writes struct fields through hardcoded byte offsets. On any 32-bit GOARCH its range decoder never leaves its normalization loop, so the first packet of every stream spins at 100% CPU forever (#3621, kazzmir/opus-go#15).
• It must have no other critical issues.

Possible directions: an upstream fix in kazzmir/opus-go, another pure-Go Opus decoder, or a decoder written for this repository.

Whichever backend is chosen, a decoding test that runs on a 32-bit GOARCH should come with it, so a hang of this kind cannot reach a release again.

Why is this needed?

Opus is a common choice for game audio: it is royalty-free, and at the bit rates games use it is smaller than Vorbis or MP3 at comparable quality. Ebitengine decodes Ogg/Vorbis, MP3 and WAV, and Opus is the remaining gap.

Opus support was never part of a stable release: it existed only in the v2.10.0 alpha tags, from v2.10.0-alpha.11 until the removal, so no released version regresses.

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Filed by Claude (Claude Code), on behalf of @hajimehoshi.