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

assets/manual/bat.1.in

@@ -263,8 +263,10 @@ Display a list of supported languages for syntax highlighting.
 .HP
 \fB\-u\fR, \fB\-\-unbuffered\fR
 .IP
-This option exists for POSIX\-compliance reasons ('u' is for 'unbuffered'). The output is
-always unbuffered \- this option is simply ignored.
+Enable unbuffered input reading. When this flag is set, bat will display data as soon as it
+is available, without waiting for a complete line. This is useful for streaming use cases like
+\&'tail \-f logfile | bat \-u \-\-paging=never'. Note that line numbers are automatically disabled
+in unbuffered mode, and syntax highlighting may be imperfect on partial lines.
 .HP
 \fB\-\-no\-custom\-assets\fR
 .IP