fix: audit Web API binding contracts

[jderochervlk]

Summary

• audit Web API binding returns and properties for spec-defined absence, using Null.t for JavaScript null and option for undefined-only values
• propagate absence through promise result types and add @throws(JsExn) to synchronous bindings that can throw
• update compile-time contract tests for the corrected nullable APIs

No audited binding required Nullable.t; the reviewed APIs distinguish null from undefined-only values.

Verification

• npm test
• npm run format:check
• npm run check:features
• git diff --check

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[jderochervlk]

@codex

[chatgpt-codex-connector]

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-08-29T14:43:22.198827Z	a11e7cf	Manual request

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: a11e7cfb79

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[chatgpt-codex-connector]

Keep HTMLElement.hidden non-nullable

HTMLElement.hidden returns a boolean or the "until-found" string; it never returns null. Wrapping the property in Null.t makes every read handle an impossible absence and requires callers to wrap ordinary boolean/string assignments. Keep this as a non-null union (or retain the previous unknown representation) in all three duplicated HTMLElement records.

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