Write Tests

Adds unit-tests for the most important non-trivial functions that do not have tests yet.

The goal is not a coverage number. The goal is that the next change to this repository breaks a test instead of breaking production. Every test written must be able to answer: which realistic defect does this test catch? If there is no answer, the test is not worth its maintenance cost.

Completion criterion

Done when:

1. The selected functions have tests that would actually fail if the behavior regressed.
2. All tests — the new ones and the pre-existing ones — run green via the repository's own test command (Step 6).
3. Nothing in the production code was changed to make a test pass (see "Non-goals"), and any bug found on the way was reported to the user rather than silently encoded as expected behavior.

Step 1 — Learn the repository's test setup before writing anything