test: cover slow-path modes that literal tests no longer reach

The buffer-at-a-time fast path now serves the literal patterns that the
existing -l/-L/-q and binary tests used, leaving the line-at-a-time
engine's equivalents uncovered. Add bracket-class (non-literal) tests
for -l/-L/-q and binary handling (notice, -a text, without-match bail,
and the finalize-time notice), plus a fast-path test for a NUL that is
only discovered after a line was already printed.

No dead code was found: the remaining uncovered lines are writer I/O
error-propagation arms and pre-existing filesystem error handlers.

tests/test_grep.rs

@@ -1339,3 +1339,103 @@ fn literal_buffer_path_spans_many_chunks() {
         .succeeds()
         .stdout_only(expected_n);
 }
+
+// Plain literals run on the buffer-at-a-time fast path, so the following tests
+// use bracket-class patterns (non-literal) to keep the line-at-a-time engine's
+// `-l` / `-L` / `-q` and binary-handling paths exercised too.
+
+#[test]
+fn slow_path_list_and_quiet_modes() {
+    let (scene, _) = ucmd();
+    scene.fixtures.write("hit", "yes\n");
+    scene.fixtures.write("miss", "no\n");
+
+    // -l: list matching files.
+    scene
+        .cmd(env!("CARGO_BIN_EXE_grep"))
+        .args(&["-l", "[y]es", "hit", "miss"])
+        .succeeds()
+        .stdout_is("hit\n");
+
+    // -L with a match in one file: only the non-matching file is listed.
+    scene
+        .cmd(env!("CARGO_BIN_EXE_grep"))
+        .args(&["-L", "[y]es", "hit", "miss"])
+        .succeeds()
+        .stdout_is("miss\n");
+
+    // -L with no match anywhere: both files listed, exit 1.
+    scene
+        .cmd(env!("CARGO_BIN_EXE_grep"))
+        .args(&["-L", "[z]z", "hit", "miss"])
+        .fails_with_code(1)
+        .stdout_is("hit\nmiss\n");
+
+    // -q stops at the first match (exit 0) or reports no match (exit 1).
+    scene
+        .cmd(env!("CARGO_BIN_EXE_grep"))
+        .args(&["-q", "[y]es", "hit"])
+        .succeeds()
+        .no_output();
+    scene
+        .cmd(env!("CARGO_BIN_EXE_grep"))
+        .args(&["-q", "[z]z", "hit"])
+        .fails_with_code(1)
+        .no_output();
+}
+
+#[test]
+fn slow_path_binary_handling() {
+    let (scene, _) = ucmd();
+    // NOTE: avoid the name "nul" here — it's a reserved device name on Windows,
+    // so writing/reading it hits the null device instead of a real file.
+    scene.fixtures.write_bytes("nulbin", b"hit\0\n");
+    scene.fixtures.write_bytes("bad", b"a\x9d\n");
+
+    // Binary notice on the line-at-a-time engine (regex pattern).
+    scene
+        .cmd(env!("CARGO_BIN_EXE_grep"))
+        .args(&["[h]it", "nulbin"])
+        .succeeds()
+        .no_stdout()
+        .stderr_contains("binary file matches");
+
+    // -a forces text mode: the NUL line is printed verbatim.
+    scene
+        .cmd(env!("CARGO_BIN_EXE_grep"))
+        .args(&["-a", "[h]it", "nulbin"])
+        .succeeds()
+        .stdout_is_bytes(b"hit\0\n");
+
+    // --binary-files=without-match bails out on an invalid-UTF-8 match.
+    scene
+        .cmd(env!("CARGO_BIN_EXE_grep"))
+        .args(&["--binary-files=without-match", "[a]", "bad"])
+        .fails_with_code(1)
+        .no_output();
+
+    // A NUL after the matched line means binariness is discovered at EOF, so
+    // the line is printed first and the notice is emitted during finalization.
+    scene.fixtures.write_bytes("late", b"hit\nno\0\n");
+    scene
+        .cmd(env!("CARGO_BIN_EXE_grep"))
+        .args(&["[h]it", "late"])
+        .succeeds()
+        .stdout_is("hit\n")
+        .stderr_contains("binary file matches");
+}
+
+#[test]
+fn fast_path_binary_detected_after_a_printed_line() {
+    // A NUL that appears only after the last match in the buffer marks the file
+    // binary on the fast path *after* an earlier match was already printed: the
+    // printed line stays and the trailing notice is still emitted.
+    let (scene, _) = ucmd();
+    scene.fixtures.write_bytes("b", b"hit\nno\0\n");
+    scene
+        .cmd(env!("CARGO_BIN_EXE_grep"))
+        .args(&["hit", "b"])
+        .succeeds()
+        .stdout_is("hit\n")
+        .stderr_contains("binary file matches");
+}