feat(#396): name every test that finished having run zero assertions

[TortoiseWolfe]

What this does

#396's last unticked item, and the one it calls highest-value: "a spec that runs zero assertions is visibly different from one that runs twelve."

Every instance in that catalogue shares one symptom — a green result that measured nothing — and every one was found by a person becoming suspicious of a specific test. This makes the symptom visible without anyone having to suspect anything first.

Not hypothetical. Two specs found yesterday guarded every assertion behind if (elements.length > 0) while visiting a page with none of those elements (#842). Fixing them immediately exposed a real 40px touch target on production. Both would have printed here as zero-assertion tests the first time they ran.

Why a reporter, not the JSON output

Checked before building it: the json reporter's result objects carry annotations, attachments, duration, errors, status, stdout — and no steps. Assertion counts are reachable only through the reporter API's step callbacks, so post-processing results.json can't do this.

It only prints

Deliberately, following this repo's own pattern of landing a gate in annotate mode first (E2E_BUDGET_MODE, FLAKY_GATE_MODE).

A zero-assertion test isn't always a defect — a spec may assert via toPass, a fixture, or a thrown helper — so failing on it today would redden the required lane for reasons nobody has triaged. Turn it into a gate once the list is known and empty.

Measured on a real batch before wiring it in: 19 passing tests across the mobile and colorblind specs, all of which asserted. The suite is in better shape than the catalogue implies, largely because #843 fixed the two that weren't — so this ships producing a clean signal rather than a backlog of noise.

It refuses to give a clean bill of health having observed nothing

If a run sees no tests — a bad shard filter, a crashed setup — it says "proves nothing" rather than "all good". A tool built to detect this family committing it would be its own #396 entry. That's asserted, not just intended.

Verification

Mutation-verified with the mutant confirmed present in the file first, three ways:

• count every step instead of only expect steps → caught
• disable the observed-nothing guard → caught
• stop recording zeros, so silent tests vanish from the map → caught (2 tests)

vitest 4724/4724 (447 files); test:scripts 424/424; type-check and lint clean.

Stated in the file so a green run isn't over-read: it cannot see assertions made by a raw throw inside a helper, and a nonzero count does not prove a test asserts anything useful.

Refs #396