Model date and time constraint validity #601

Parent: #257. Tracking epics: #235 and #227.

Complete the next bounded form-state slice for date, month, week, and time inputs using live control state rather than content-attribute approximations.

Scope

• strict parsing and canonical numeric conversion/serialization;
• type-specific valueAsNumber scales, step defaults, bases, min/max, and mismatch handling;
• reversed periodic time ranges and transient native bad input;
• reuse compiled live/attribute dependency invalidation without document-wide recascade;
• add WPT-derived browser contracts, native source contracts, a candidate profile entry, and an explicit deferred-validation note.

Validation truth

git diff --check passed after rebasing to current main. Authored contracts, builds, WPT, benchmarks, product acceptance, and computer-use remain deferred.