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/controller.rs

@@ -158,6 +158,7 @@ impl Controller<'_> {
             #[cfg(not(feature = "lessopen"))]
             input.open(stdin, stdout_identifier)?
         };
+        opened_input.reader.unbuffered = self.config.unbuffered;
         #[cfg(feature = "git")]
         let line_changes = if self.config.visible_lines.diff_mode()
             || (!self.config.loop_through && self.config.style_components.changes())
@@ -327,6 +328,9 @@ impl Controller<'_> {
                     }
 
                     printer.print_line(false, writer, line_nr, &line, max_buffered_line_number)?;
+                    if self.config.unbuffered {
+                        writer.flush()?;
+                    }
                 }
                 RangeCheckResult::AfterLastRange => {
                     break;