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/output.rs

@@ -236,4 +236,11 @@ impl OutputHandle<'_> {
             Self::FmtWrite(handle) => handle.write_fmt(args).map_err(Into::into),
         }
     }
+
+    pub fn flush(&mut self) -> Result<()> {
+        match self {
+            Self::IoWrite(handle) => handle.flush().map_err(Into::into),
+            Self::FmtWrite(_) => Ok(()),
+        }
+    }
 }