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/bin/bat/app.rs

@@ -462,6 +462,7 @@ impl App {
                 _ => unreachable!("other values for --strip-ansi are not allowed"),
             },
             quiet_empty: self.matches.get_flag("quiet-empty"),
+            unbuffered: self.matches.get_flag("unbuffered"),
             theme: theme(self.theme_options()).to_string(),
             visible_lines: match self.matches.try_contains_id("diff").unwrap_or_default()
                 && self.matches.get_flag("diff")
@@ -619,6 +620,11 @@ impl App {
             bat_warning!("Style 'rule' is a subset of style 'grid', 'rule' will not be visible.");
         }
 
+        // Auto-disable line numbers in unbuffered mode to avoid confusion with partial lines
+        if self.matches.get_flag("unbuffered") {
+            styled_components.0.remove(&StyleComponent::LineNumbers);
+        }
+
         Ok(styled_components)
     }