feat: add Opus streaming encoder (item 8's Opus sub-item)

[cloud-hai-vo]

Summary

• Implements the Opus sub-item of docs/streaming-plan.md item 8: OpusStreamingEncoder is the second IStreamingEncoder implementation, reusing OpusEncoderSession's own Concentus-backed encode internals unchanged — only how each encoded packet is delivered differs (a callback emitting raw packets instead of Ogg-paged bytes to a file), mirroring the exact approach PR feat: add push-style IStreamingEncoder + AAC-LC streaming session #110 established for AAC-LC.
• Two differences from AacStreamingEncoder, both because Opus's own codec characteristics genuinely differ from AAC-LC's:
  • EncoderDelaySamples is a per-instance property reading the underlying encoder's own Lookahead directly, since Opus's lookahead isn't architecturally fixed at one value the way AAC-LC's one-MDCT-frame delay is.
  • Every emitted frame's own SampleCount is always the full 960 — Opus has no bitstream-level short-final-frame concept (every encoded Opus packet is exactly one fixed frame duration); the real-vs-padding distinction for a partial tail only matters at the container level, out of scope for this raw packet-level encoder.
• Complete() drains the encoder's own fixed lookahead through the normal buffering path (the only way to push the genuine tail of real input out of the encoder's internal state), but — unlike OpusEncoderSession.Finish() — does not unconditionally emit one final all-zero frame when nothing remains pending afterward; that behavior exists in the file-based session purely to flag an Ogg page's own end-of-stream bit, a container-level concept this raw-packet encoder has none of.
• MP3 and FLAC streaming sessions (the other two sub-items of item 8) remain not started.
• Docs synced: docs/streaming-plan.md (Opus sub-item marked done, item 8 overall marked partially done), CLAUDE.md, README.md, llms.txt, llms-full.txt.

Test plan

• New OpusStreamingEncoderTest: construction validation (null callback, unsupported channels/sample rate), every frame including the final one reports SampleCount == 960, monotonic frame indices, Complete() with zero samples emits nothing, idempotent Complete()/safe double Dispose(), Write after Complete() throws, and a real round-trip SNR test (both mono and stereo) that reconstructs a full OggOpus file from the streaming encoder's own raw packets (via the existing internal OggPageWriter/OpusHeaderPackets) and decodes it through the project's own public OpusDecoder — proving the raw packets are genuinely valid Opus without new decode-side test infrastructure.
• Full existing suite (dotnet test, net10.0 locally): 1686 passed, 1 pre-existing unrelated skip, 0 failed.
• Full solution build (all target frameworks + AotSmokeTest): 0 warnings/errors.

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