CakeML checkers should call Runtime.exit with error codes

They currently return 0 in all cases (except heap/stack memouts, which exit with 1 and 2 respectively)

Start by locating the CakeML checker implementations and their existing Runtime.exit calls, then inspect the return paths for normal failures and the heap/stack memout cases. Determine the intended error-code mapping from the surrounding checker behavior or project guidance, and verify that each relevant failure exits with the expected code while the existing memout codes remain correct.

Written by the indexing model from the issue text.