feat: typed custom events between form and fields

[LouisHaftmann]

Summary

Prototype for a bidirectional, fully-typed custom event channel between a form and its fields, built on top of the existing hookable instance.

• field → form: field.events.callHook(name, payload) emits; form.events.hook(name, (payload, field) => …) listens and receives the sender field.
• form → field: form.events.callHook(name, payload) emits (broadcast to all field listeners); field.events.hook(name, payload => …) listens.
• Event payloads are declared app-wide via a module-augmentable Register interface, keyed by direction:

declare module '@falcondev-oss/form-core' {
  interface Register {
    events: {
      toForm: { validated: { ok: boolean }; requestScroll: void } // field → form
      toField: { flash: { color: string } }                        // form → field
    }
  }
}

Directions are type-enforced (a form can't emit a toForm event, and vice versa), unknown events / wrong payloads are rejected, and void events take no payload. hook/hookOnce/callHook naming mirrors hookable.

Implementation

• One extra createHooks() bus per form (separate from lifecycle hooks); names namespaced by direction (toForm: / toField:).
• events facade added to the form handle and to each FormField.
• useFormHandles (vue) aggregates events across forms.

Tests

• Runtime (core.test.ts): field→form w/ sender, form→field, broadcast, void payload, direction isolation.
• Types (types.test-d.ts): typed payloads + sender, direction enforcement, unknown/wrong/missing payloads rejected.

Type-check ✓, lint ✓, tests green (only the pre-existing field > readonly failure remains, unrelated).

Note: .patch(partial, metadata) is designed but not in this PR.