[gouyt13]

Summary

Strengthen pre-main Core and FastScan tests so silent no-ops or incorrect finite results cannot pass. Use independent nonzero distance references, sentinel output buffers, and known LUT/HACC results.

Add --no-tests=error to the 11 remaining unguarded CTest invocations and a workflow-contract test covering all 16 commands plus an actual empty-test-directory failure. Production library behavior is unchanged.

Testing

• Correct implementations pass both startup tests. Four temporary startup-only fault controls (LUT generation, HACC transfer, accumulation, and Core zero results) are reached and rejected; these mutations are not committed.
• New CI contract test fails on the old workflow and passes after the guards are added.
• Portable CTest: 406 passed, 1 skipped; the placement test passed separately with OpenMP binding enabled.
• CI helper suite: 57 passed, 502 subtests passed. C++ formatting, Python lint/format, workflow YAML parsing, and git diff --check passed.

Validation was local Linux; hosted Windows/macOS/ARM jobs have not run this patch yet.