test: add a CLI --help integration test

Runs the built binary with --help and asserts it exits 0 and prints the usage, options, and about text (visible via cargo test -- --nocapture).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/cli.rs

@@ -0,0 +1,24 @@
+//! End-to-end CLI tests that run the actual built binary.
+
+use std::process::Command;
+
+/// `--help` must exit successfully and print the usage/options. Run with
+/// `cargo test -- --nocapture` to see the help text in the test output.
+#[test]
+fn help_shows_usage_and_options() {
+    let output = Command::new(env!("CARGO_BIN_EXE_fsdecrypt"))
+        .arg("--help")
+        .output()
+        .expect("failed to run fsdecrypt --help");
+
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    println!("\n--- fsdecrypt --help ---\n{stdout}");
+
+    assert!(output.status.success(), "--help should exit 0");
+    assert!(stdout.contains("Usage"), "help should show a usage line");
+    assert!(stdout.contains("--no-extract"), "help should list options");
+    assert!(
+        stdout.contains("decryptor for some SEGA containers"),
+        "help should show the about text"
+    );
+}