Add `field.linkedPair()` to bake cross-field links into the schema

[akladekouassi]

Summary

Add field.linkedPair(label) — a composite that wraps two related sub-fields with a built-in linkage rule (e.g. password + confirm, or country + region cascade). Removes the repetitive cross-validation noise that today lives in every schema.

Why this would help

• Password confirmation, region cascading from country, "current value vs new value" change forms, "from / to" semantic pairs.
• Keeps the linkage rule next to the fields it relates to instead of scattered across validators.
• The two sub-fields keep their own typed APIs, so nothing breaks reusability.

Proposed API

field.linkedPair('Password')
  .pair(
    field.password('Password').required().strong(),
    field.password('Confirm').sameAs('password'),
  );

field.linkedPair('Location')
  .cascade(
    field.country('Country'),
    field.select('Region').optionsFrom((country) => loadRegionsFor(country)),
  );

Suggested implementation areas

• src/core/field-descriptors/linked-pair/LinkedPairFieldBuilder.ts (new)
• src/core/field-descriptors/field.ts
• generic linkage protocol: pair(a, b) adds cross-validation, cascade(parent, child) adds child option re-derivation on parent change
• src/renderers/web/ and src/renderers/native/ — render the two sub-fields stacked / inline depending on layout option

Acceptance Criteria

• pair() runs the linkage validator on every change of either sub-field
• cascade() re-derives the child's options when the parent value changes
• The composite stores the two values under sub-keys (e.g. password.value, password.confirm) — final shape is documented
• Tests for both pair() and cascade() with realistic schemas (password sameAs, country/region)

[github-actions]

Mirrored to monorepo issue: https://github.com/runilib/runilib/issues/188