feat: add TTA as a Stream-to-Stream ConvertFile/CutFile destination

[cloud-hai-vo]

Summary

Continues docs/feature-roadmap.md item 1's own "destination-Stream" half (WAV/AIFF/AU/FLAC already done — see #108, #122). This item: TTA.

• TtaEncoderSession.OpenSession(Stream destStream, ...) -- a new overload, mirroring WavWriter/AiffWriter/AuWriter/FlacEncoder's own Create(Stream, ...)/OpenSession(Stream, ...) overloads. Needs no Stream.CanSeek at all -- unlike FLAC (the only prior destination that genuinely requires it), TTA already holds every compressed frame in memory until Finish() (TtaEncoderSession's own pre-existing design, since TtaWriter's own seek table needs every frame's final size up front regardless), at which point TtaWriter.Write writes the header, seek table, and frame data in a single forward pass with no patch-up ever needed. Confirmed directly via a test that writes through a destination whose Seek/Position throw, not just assumed from the absence of a CanSeek check.
• TtaEncoderSession's own destination field was widened from FileStream to Stream plus a leaveOpen flag (mirroring WavWriter's own convention) so Dispose() only closes a stream this type actually owns.
• Channel/bit-depth/sample-rate validation factored into a shared private helper so the path-based and Stream-based OpenSession overloads can't drift apart on what's valid.
• Wired into AudioCutter.Stream.cs's OpenSinkStream as the AudioFormat.Tta destination case -- no DeferredFixedHeaderSink needed, same reasoning as FLAC's own case.

Docs synced: CLAUDE.md, README.md, llms.txt, llms-full.txt, docs/feature-roadmap.md item 1's own status note.

Test plan

• New TtaEncoderSessionTest cases: null stream, unsupported channel count/bit depth/sample rate, exact round-trip via a MemoryStream destination, a genuinely non-seekable destination (confirms no CanSeek requirement end-to-end, not just an absent check), and Dispose() not disposing the caller's stream.
• New AudioCutterStreamTest cases: WAV source → TTA Stream destination round-trip (exact samples, lossless), and a non-seekable TTA destination succeeding through the full AudioCutter.Convert call.
• dotnet test --configuration Release (net10.0 locally): 1759 passed (1750 baseline + 9 new), 2 pre-existing skips (1 WMA, 1 the already-tracked MP3 synthesis-filter bug), 0 failed.
• dotnet build --configuration Release: 0 warnings, 0 errors, all target frameworks.

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