Fix Input validation, value handling and number steppers

Input:
- honour `invalidMessage` instead of always reporting "Value mismatch"
- don't let the mount value wipe an external `validity` prop, and don't
  force `isValid: true` on clear when `validity` is supplied
- sync controlled and programmatic values into `textarea` (the value
  effect had an empty branch that skipped textareas entirely)
- accept `defaultValue`/`value` of `0` and `""` (nil check, not truthiness)
- clear debounce/validation/change-handler timers on unmount
- type `wrapperProps` as `ComponentProps<"div">`
- let `labelProps.htmlFor` override the name link deterministically
- drop duplicated focus outline classes and the stale buttonDown guard
  that discarded edits made right after holding a stepper

NumberInputButtons:
- write through `setValue` so steppers work on controlled inputs and
  fire `onChange`
- clamp to both `min` and `max`, including `0` values, and respect `step`
- round float drift, ignore steps while a trailing decimal point is typed
- instance-scoped press timers with unmount cleanup; a held press no
  longer double-steps on the trailing click

Also adds a numberText stepper example to the dev playground.

dev/src/pages/forms.tsx

@@ -76,6 +76,10 @@ export default function FormsPage() {
                         prefix="$"
                         suffix="USD"
                         inputMode="decimal"
+                        numberText
+                        min={50}
+                        defaultValue={50}
+                        step={5}
                     />
                     <Input
                         label="Error state"