Model numeric constraint validity #597

Parent: #257.

Problem

Number and range controls need strict live numeric parsing and constraint state. Bad input, range underflow/overflow and step mismatch must use type defaults/bases, normalized range values and willValidate gating; live value and min/max/step changes must invalidate only compiled dependent selectors.

Implemented boundary

• Strictly parse number/range live values and expose live valueAsNumber.
• Compute bad input, range underflow/overflow and step mismatch.
• Apply type-specific default step, step base and range normalization.
• Honor validation gating and live/dirty value state.
• Add compiled min/max/step invalidation routes.
• Add WPT-shaped/native source contracts and validation/deferred documentation.

Deferred to #257

Date/time/month/week constraints, constraint-validation IDL, custom validity, check/report validity, submit-attempt user validity, form/fieldset aggregation and remaining external-owner behavior.

Validation boundary

git diff --check passed. Authored contracts and generated binding validation were not executed under the current implementation-throughput direction.