test: widget gate parity pin — client gate ⇔ server validators (architecture review F2)

[silversurfer562]

Summary

Batch B of the 2026-08-20 dynamic-forms architecture review: the one finding with live user-facing drift risk (F2).

The widget submit script's client-side required-field gate (widget.py:865-923) re-implements the server validators' completeness rules — required rulings boards fully ruled, ranking all-or-nothing, blank edit text blocked, optional partial rankings blocked — and nothing in CI kept the two implementations in sync. A divergence would either let an invalid answer post after the widget is dead (nowhere to re-ask) or block a valid one.

What this adds (tests only, no behavior change)

• Gate port: the gate's two passes ported rule-for-rule into tests/test_widget_roundtrip.py (_gate / _gate_allows), operating on the same _WidgetDOM simulator the reader pin uses. The simulator now also captures data-required and counts [data-item] rows — the anchors the JS gate queries.
• Parity property test: 35 fixtures (every construct × empty/partial/valid fill states, plus optional-ranking and blank-edit edges) asserting gate blocks ⇔ collect_form_response rejects. No fixture hardcodes a verdict — the property is the assertion, so it fails whichever side drifts.
• Reference-form parity: untouched (both block) and fully-filled (both pass) checks over the all-construct reference form.
• Structural anchor drift catcher: the DOM attributes the gate JS queries (data-required, [data-item], data-rank-n, .ae-rank, ae-field-missing) must appear in the emitted script, and the renderer's data-required flags must match the form's required questions — same technique as the reader's collect-mode pin at test_script_switch_covers_the_emitted_collect_modes.

Verification

• Full suite: 717 passed.
• Mutation check: disabling the blank-edit rule in the port fails assume-blank-edit red.

Stated limitation (as in the review): this pins semantic agreement (Python gate model ⇔ server validators) plus script structure — not JS execution. A node/playwright CI runner for one script was rejected as disproportionate; a real human submit stays the gold receipt.

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