Validate forms and aggregate control validity at submission #605

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

Aggregate control validity at form/fieldset boundaries and gate every supported submission path through the merged constraint-validation API.

Scope

• live form-owner and fieldset-descendant validity aggregation;
• form checkValidity() and reportValidity() with per-control invalid dispatch;
• implicit, programmatic, native-pointer, and semantic submit activation;
• submit-attempt user-validity transitions for owned input/select/textarea controls;
• honor novalidate and submitter formnovalidate;
• block invalid submission without introducing interactive validation UI;
• browser/native/profile/deferred-validation contracts.

Deferred scope

Descendant-driven ancestor computed-style recascade, interactive validation UI, requestSubmit()/submit(), dedicated fieldset IDL, datetime-local/file gaps, and UI-only bad-input/user-length transitions remain separate.

Validation truth

git diff --cached --check passed. Builds, WPT, benchmarks, product acceptance and computer-use remain unexecuted.