feat!: array-element field identity via @solidjs/signals store

[LouisHaftmann]

Closes #4.

What & why

Replaces @vue/reactivity and the hand-written on-change array-mutation mirror with the @solidjs/signals store (SolidJS 2.0, next) as form-core's reactive engine. Array-element identity is now maintained by the engine, so a FormField travels with its datum: reorder / splice / sort / reverse / unshift / move keep each element's key, dirty state, and errors attached to the element, not the index. This removes the fragile per-method mirror the issue set out to kill.

Key changes

• Field cache re-keyed by store-node identity (WeakMap<node, { self, children }>). Primitive-leaf array elements stay positional (the accepted hybrid). Deleted the nested _array cache, getFieldCachePath, the on-change onValidate/onChange mirror, and the data-path → cache-path transform.
• Writable form.data facade — a thin path-tracking proxy over the store; reads pass through (reactive, identity-stable), writes route into the engine setter. New form.setData(recipe) applies a batch atomically with a single validation.
• Asynchronous flush contract — writes are microtask-batched; value-level derived state is current only after the next flush. No public settled promise; a hidden form['~'].flush() escape hatch forces the pending batch current (used by imperative/test code and the framework bindings).
• Whole-form validation driven by a single deep(store) effect (replacing the on-change global callback); per-field issue filtering by path is unchanged; on-change/on-blur/on-submit timing preserved.
• reset() / non-dirty sourceValues updates use reconcile — positional by default (reproduces prior reset behavior, keeps fields stable), with an optional per-form reconcileKey resolver to preserve identity across an id-keyed external refresh. Dirty-guard unchanged.
• Adapters rewritten. React bridges the store into re-renders via useSyncExternalStore; Vue bridges store reactivity into Vue and keeps field.model as a writable v-model computed. Migrated the React hook tests off the React-19-incompatible @testing-library/react-hooks.
• Data purity preserved (snapshot() feeds validation/submit); field.schema, translate, discriminator, hooks unchanged.

Implementation note: identity + the beta store

The beta store clones an element that still carries a pending write-override when its array is reordered, which would sever the field's identity after an edit. To honor the headline user story (drag-reorder of edited rows keeps focus/dirty), each array-restructuring op first bakes the store (reconcile(snapshot(store))) to collapse the override layer — transparent to values and node identity. It's an O(n) step at form scale, pinned to @solidjs/signals@2.0.0-beta.24; the pre-release dependency and its blast radius were accepted in the issue.

Breaking changes (pre-1.0)

• form-core now depends on @solidjs/signals instead of @vue/reactivity; the @falcondev-oss/form-core/reactive entrypoint was removed.
• Read/write contract is asynchronous — imperative code must await a tick or call form['~'].flush().
• field.key's string format changed; array.delete(key) resolves the element by identity instead of parsing an index.

Tests

Core (49), React (2), Vue (2) all green; tsc --build, ESLint/Prettier, and pnpm build clean. Identity is asserted through the public handle only (same key, preserved dirty/errors, correct at(i) after a move), including the edit-then-reorder case; primitive-array positional behavior, delete(key) after reorder, setData single-validation, reset, dirty-guard, and the key resolver are all covered.

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