The `min` and `max` validation rules previously handled `string` values
to accommodate numbers bound to text inputs. However, this is no longer
necessary as the control binding itself handles the conversion.

This change removes string support from these rules, simplifying the
types to `number | null`. The validation logic has been updated to use
concrete checks (`value === null || Number.isNaN(value)`) to ensure safe
TypeScript narrowing.

Associated tests have been updated to:
- Remove string-specific validation checks.
- Add coverage for text input bindings.
- Add coverage for empty input handling (standard behavior where empty
  sets model to null and skips validation).

BREAKING CHANGE: `min` and `max` validation rules no longer support
string values. Bound values must be numbers or null.

- Added `minDate()` and `maxDate()` for validating constraints on `Date` inputs.
- `ReadonlyFieldState.min` and `.max` now return
  `Signal<NonNullable<TValue>`. This ensures that `min` and `max` inputs
  on custom controls can accept a reliable type (matching their value
  type).
- Made the `TWrite` type parameter of `MetadataKey` contravariant to
  properly indicate that it's writable.
- Added `LimitKey` as a convenience type for defining validation limit
  metadata (e.g. `MAX_NUMBER`, `MIN_DATE`).
- Added `LimitSelectionKey` which can be used to bind a `LimitKey` with
  value-specific aggregation logic, to a generic metadata key (e.g. use
  `MAX_NUMBER` to aggregate numbers for `MAX`).

… `minDate`/`maxDate`