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

tests/integration_tests.rs

@@ -3701,3 +3701,39 @@ fn cache_help_shows_help_message() {
         .stdout(predicate::str::contains("--build"))
         .stdout(predicate::str::contains("--clear"));
 }
+
+#[test]
+fn unbuffered_flag_is_accepted() {
+    bat()
+        .arg("--unbuffered")
+        .arg("test.txt")
+        .assert()
+        .success()
+        .stdout("hello world\n");
+}
+
+#[test]
+fn unbuffered_mode_disables_line_numbers() {
+    // When --unbuffered is used, line numbers should be auto-disabled even if requested
+    bat()
+        .arg("--unbuffered")
+        .arg("--style=numbers")
+        .arg("--decorations=always")
+        .arg("--color=never")
+        .arg("test.txt")
+        .assert()
+        .success()
+        .stdout(predicate::str::starts_with("   1").not());
+}
+
+#[test]
+fn unbuffered_mode_plain_output() {
+    bat()
+        .arg("--unbuffered")
+        .arg("--color=never")
+        .arg("--decorations=never")
+        .arg("test.txt")
+        .assert()
+        .success()
+        .stdout("hello world\n");
+}