feat: add FLAC streaming encoder, completing item 8

[cloud-hai-vo]

Summary

• Implements the FLAC sub-item of docs/streaming-plan.md item 8 — the last of its three codec sub-items, now that the pattern has been proven three times over for AAC/Opus/MP3 (PRs feat: add push-style IStreamingEncoder + AAC-LC streaming session #110, feat: add Opus streaming encoder (item 8's Opus sub-item) #118, feat: add MP3 streaming encoder (item 8's MP3 sub-item) #120). FlacStreamingEncoder is the fourth IStreamingEncoder implementation, reusing FlacFrameEncoder's own encode internals unchanged.
• Genuinely simpler than the other three, for two real codec-level reasons, not a shortcut: this encoder's own FIXED predictors are purely causal (never look at "future" samples), so there is no encoder lookahead to drain at all (EncoderDelaySamples is always 0); and FLAC's own frame format natively allows a final block shorter than the nominal size with no padding, unlike AAC-LC/Opus/MP3's own fixed-or-padded frame sizes. Complete() therefore just flushes whatever's left pending, nothing more.
• Genuinely not independently decodable, unlike the other three: FlacFrameEncoder's own existing frame header always defers sample-rate and bits-per-sample to STREAMINFO (a pre-existing characteristic of this project's frame encoder, not introduced here) rather than using FLAC's own available explicit escape codes. Building a STREAMINFO (a whole-stream, container-level concept: total duration, a whole-stream MD5, min/max block size) is out of scope for this raw packet-level encoder, the same boundary every other IStreamingEncoder already draws.
• Widened FlacEncoderSession.WriteStreamInfoPayload from private to internal (zero behavior change, confirmed by the full existing FlacEncoderTest suite still passing unchanged) so this item's own conformance tests can build a real STREAMINFO out-of-band and reuse that exact bit layout rather than duplicating it.
• Conformance verifies exact sample equality after a full decode (via the project's own FlacDecoder), not an SNR floor — the same bar every lossless codec in this project is already held to, unlike the lossy AAC/Opus/MP3 sub-items.
• This completes docs/streaming-plan.md item 8 in full.
• Docs synced: docs/streaming-plan.md (FLAC sub-item + item 8 overall both marked done), CLAUDE.md, README.md, llms.txt, llms-full.txt.

Test plan

• New FlacStreamingEncoderTest: construction validation (null callback, zero channels, out-of-range bits-per-sample both directions), SamplesPerFrame == 4096, Complete() with zero samples emits nothing, idempotent Complete()/safe double Dispose(), Write after Complete() throws, monotonic frame indices, exact short-final-block sample count with no padding, the "exactly one full block, nothing else emitted" edge case (no lookahead to drain, unlike Opus/MP3), and exact (not SNR-based) round-trip for mono and stereo via a real decoded FLAC file built from the streaming encoder's raw frames plus an out-of-band STREAMINFO.
• Full existing suite (dotnet test, net10.0 locally): 1718 passed, 1 pre-existing unrelated skip, 0 failed.
• Full solution build (all target frameworks + AotSmokeTest): 0 warnings/errors.

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