The structural test conventions fail a run rather than a review

## User story

As a **contributor adding a test module**, I want **the conventions a machine can check to be checked**, so that **a missed one fails a run instead of waiting for a reviewer to spot it, and the prose stops carrying rules a tool could hold**.

## Current workaround

Three testing conventions are stated in prose and enforced by attention alone. Each has already drifted.

- **A test module mirrors its subject**, `_test` appended to every path segment. Nothing checks the mapping in either direction, so a module can gain code with no test module, and a test module can outlive the module it mirrors.
- **Every test package carries an `__init__.py`.** `cli_test/` and `task_test/` have gone without since before the convention; PR #621 adds them. Nothing stops the next directory landing without one, and the missing files are visible today as pytest collecting `cli_test/main_test.py` twice.
- **The module under test is imported as `sut`.** Half the suite does otherwise; #679 covers settling that.

The prose is the weakest place for a rule a tool could hold. It costs a reader on every visit, it cannot fail a pull request, and it drifts silently from what the suite does.

## Alternatives considered

- A `pre-commit` hook per check, matching how `vale`, `vulture`, and the FawltyDeps hooks already sit in `.pre-commit-config.yaml`.
- One script under `.github/scripts/`, called from a hook, the way `mutation-summary.sh` is called from the nightly job.
- A `ruff` custom rule, which covers the import alias but not the path mapping.
- Leaving the mirror check out: a module that genuinely warrants no test module would need an opt-out, and the list of those may be long enough to make the check noise.

## Acceptance criteria draft

- [ ] A check fails when a module under `zfs/` has no mirroring `_test` module, or a test module mirrors nothing, with an opt-out for the deliberate cases.
- [ ] A check fails when a directory under `zfs_test/` lacks `__init__.py`.
- [ ] Each check runs from `pre-commit` and in CI, and fails a pull request rather than reporting.
- [ ] Prose that only restated a now-checked rule is deleted rather than left beside the check.

## Additional context

- #679 settles the import alias, which has to land before a check for it can go on.
- #677 asks for exactly this: rules a linter could carry get filed for enforcement rather than restated.
- PR #621 fixes today's missing `__init__.py` files.