go-testing

# Go Testing Tests are executable specifications. Their job is to **fail usefully** when behaviour regresses — and to keep failing in the same way until the bug is fixed. Tests that are passing-or-flaky teach the team to ignore them, which is worse than no test at all. ## Core Rules 1. **Failures must be diagnosable from the log alone.** Every `t.Errorf` includes the function under test, the inputs, what we got, and what we wanted, in that order. 2. **No assertion libraries by default.** Use the standard `t.Errorf` / `t.Fatalf` plus `go-cmp` for structural comparison. `testify` is acceptable when adopted consistently — pick one and stick with it. 3. **Test observable behaviour, not implementation details.** If a refactor that preserves behaviour breaks the test, the test was wrong. 4. **Each test runs independently.** No execution-order dependencies, no shared global state without `t.Cleanup`. 5. **`t.Parallel()` whenever the test is safe to run in parallel.** Most are. 6. **`t.Helper()` is the first line of any helper function** that calls `t.Errorf`/`t.Fatalf`. Reserve `t.Fatal` for "next line is meaningless without this value"; everything else uses `t.Error`. Never call `t.Fatal`/`t.FailNow` from a non-test goroutine — send the failure back via channel. ## "Useful Failures" Format The failure message is the test's user interface. The canonical shape: ``` FunctionUnderTest(input) = got, want want ``` ```go // Good t.Errorf("Add(2, 3) = %d, want %d", got, 5) // Bad —...