Function read_line 

pub fn read_line(
    reader: &mut impl BufRead,
    maximum: u64,
) -> Result<Vec<u8>, FrameReadError>

Read one frame, retaining its newline and leaving later frames buffered.

maximum includes the newline. At most one excess byte is consumed to detect overflow; this bounds retained bytes, not allocator capacity or elapsed time. The caller owns parsing, deadlines, thread/process lifetime and diagnostics. A refusal does not resynchronize the channel; callers must stop it or choose their own recovery policy.

Errors

Distinguishes clean EOF, missing newline, overflow and original I/O failures.