feat: implement --unbuffered mode for streaming input (#3555)

Repurpose the existing --unbuffered/-u flag (previously a POSIX no-op)
to enable unbuffered input reading using fill_buf()/consume() instead
of read_until(b'\n'). This allows partial lines to display immediately
when piping streaming input like `tail -f` into bat.

- Add unbuffered field to Config and InputReader
- Add read_line_unbuffered() using BufRead::fill_buf()/consume()
- Add flush() to OutputHandle, called after each line in unbuffered mode
- Auto-disable line numbers in unbuffered mode to avoid partial line confusion
- Update help text, man page, and shell completions
- Add unit tests and integration tests

src/config.rs

@@ -110,6 +110,9 @@ pub struct Config<'a> {
 
     /// Whether or not to produce no output when input is empty
     pub quiet_empty: bool,
+
+    /// Whether or not to use unbuffered input reading for streaming use cases
+    pub unbuffered: bool,
 }
 
 #[cfg(all(feature = "minimal-application", feature = "paging"))]