craft-audio: FLAC encoder

FilmCraft's FLAC encoder behind `flac-encode`, kept over SoundCraft's:
linear prediction, stereo decorrelation and the STREAMINFO MD5 give
0.1-37% smaller files on 12 test signals, and the output is bit-exact
through `flac` and ffmpeg. It is 3-5x slower than SoundCraft's, still
about 56x real time.

`lossless-encode` is now `pcm-encode` plus `flac-encode`, and
`EncodeFormat::Flac` writes it through `encode` and `FileEncoder`.
With only one of the two features, the other's formats are refused as
`Unsupported`.

The tests that use FLAC files as decode fixtures (truncation,
corruption, metadata bounds, zero-frame files, container preflight)
come with it, as do the FLAC oracle tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/audio.yml

@@ -35,7 +35,7 @@ jobs:
       # Each feature on its own, so code that needs another feature fails here.
       - name: Clippy, each feature alone
         run: |
-          for f in "" decode decode-native symphonia encode lossless-encode pcm-encode peaks serde; do
+          for f in "" decode decode-native symphonia encode lossless-encode pcm-encode flac-encode peaks serde; do
             echo "::group::features: ${f:-none}"
             cargo clippy -p craft-audio --all-targets --locked --no-default-features ${f:+--features "$f"} -- -D warnings
             echo "::endgroup::"
@@ -82,6 +82,7 @@ jobs:
           key: craft-audio-features
       - run: cargo test -p craft-audio --no-default-features --features lossless-encode --locked
       - run: cargo test -p craft-audio --no-default-features --features pcm-encode --locked --lib
+      - run: cargo test -p craft-audio --no-default-features --features flac-encode --locked --lib
       - run: cargo test -p craft-audio --no-default-features --features decode-native --locked
   test:
     timeout-minutes: 20