test(storybook): prove the autoplay contract the smoke harness stands on

[UncertaintyDeterminesYou4ndMe]

Summary

The remaining item from #1981. The doc half was already fixed on main — FIDELITY.md's play section now correctly states that plays execute and a throwing play fails the smoke run. What was still missing is the guard the issue asked for: that failure path only exists while plays actually run. If a Storybook upgrade turned autoplay off, throwing plays would stop failing, silently.

A literally-throwing story can't ship (it would keep CI permanently red), so the contract is proven positively instead:

• harness-play-contract.stories.tsx (Design System/Harness Contracts, exempt namespace): renders a data-play-proof="pending" node; its play flips it to executed.
• A new play-executed product check in storybook-visual-smoke.mjs fails the run when the marker never flips, with a message that names the broken contract.
• The surface joins REQUIRED_PRODUCT_SURFACES, so it cannot be switched off by deleting its manifest entry — the same durability rule the other required surfaces get.
• FIDELITY.md's play section points at the contract.

Closes #1981.

Verification

Both directions, locally against a real build-storybook output:

• Positive: smoke:storybook passes — Product Storybook smoke passed (68 manifest check(s), 73 catalog render(s)).
• Negative (the actual acceptance): with the play sabotaged into a no-op (marker never flips, simulating an autoplay regression), the run exits 1 with [design-system-harness-contracts--play-executes @ wide] play function did not execute — the Storybook autoplay contract (FIDELITY.md) is broken. Restored and re-verified green after.
• tsc -p tsconfig.storybook.json, biome format/lint, check-story-annotations all clean.

[Astro-Han]

Approve. Two follow-ups for the record, neither blocking:

• The guard is only as durable as its manifest entry: deleting the surface's checks field passes validateCoverageManifest, so the guard can vanish silently. A REQUIRED_PRODUCT_CHECKS map (surface to required checks) would pin the wiring the way REQUIRED_PRODUCT_SURFACES pins the surface.
• The negative direction is verified manually only. The predicate lives inside a page.evaluate string; extracting it as an exported pure function would make it unit-testable in Node and catch a sabotaged predicate before it ships.