Demonstrates TEST_CASE, CHECK, and REQUIRE with arithmetic, string, and container checks.

Demonstrates CHECK_THROWS, CHECK_THROWS_AS, CHECK_CATCHES_AS, and CHECK_NOTHROW using a small parse_int function that throws on bad input.

Shows how CHECK and REQUIRE can be called from helper functions outside the TEST_CASE body. On failure the framework prints a stacktrace of user-code frames so the call site is easy to find.