craft-audio: review fixes, round 2 (#442)

- check_room docs: FLAC holds at most 2^36 - 1 frames.
- The FLAC round trip covers every channel count from 1 to 8.

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

crates/audio/src/write/flac/mod.rs

@@ -75,7 +75,7 @@ impl<W: Write + Seek> Writer<W> {
         })
     }
 
-    /// The frame count after `frames` more, or an error past what FLAC can count (2^36 frames).
+    /// The frame count after `frames` more, or an error past what FLAC can count (2^36 - 1 frames).
     pub(crate) fn check_room(&self, frames: u64) -> Result<u64> {
         let total = self.frames.saturating_add(frames);
         if total >= 1u64 << 36 {

crates/audio/src/write/mod.rs

@@ -175,7 +175,7 @@ impl<W: Write + Seek> FileEncoder<W> {
     }
 
     /// Fails, without writing anything, when `frames` more frames would not fit the format (an
-    /// AIFF holds 4 GiB, a FLAC 2^36 frames).
+    /// AIFF holds 4 GiB, a FLAC 2^36 - 1 frames).
     pub fn check_room(&self, frames: u64) -> Result<()> {
         match &self.writer {
             #[cfg(feature = "pcm-encode")]

crates/audio/tests/audio/roundtrip.rs

@@ -73,7 +73,7 @@ fn wav_and_aiff_roundtrip_all_depths_and_layouts() {
 #[test]
 fn flac_roundtrip_is_lossless_vs_wav() {
     for depth in [BitDepth::Int16, BitDepth::Int24, BitDepth::Int32] {
-        for channels in [1, 2, 6] {
+        for channels in [1, 2, 3, 4, 5, 6, 7, 8] {
             // Spans several blocks plus a short final block.
             let src = sine(44_100, channels, 4096 * 2 + 17);
             let wav = encode(&src, &EncodeOptions::new(EncodeFormat::Wav).with_bit_depth(depth).with_dither(false)).unwrap();