Function write_consolidated_frames 

pub async fn write_consolidated_frames<F: VfsFile>(
    cx: &Cx,
    wal: &mut WalFile<F>,
    batches: &[TransactionFrameBatch],
) -> Result<usize>

Write a consolidated batch of frames to the WAL file.

Serializes all frames into a single contiguous buffer and writes it in one write call, then fsyncs. This amortizes syscall overhead and ensures all frames in the group become durable atomically.

Updates the WAL file’s running_checksum and frame_count for each frame in the batch, maintaining the checksum chain invariant.

Returns the number of frames written.