feat!: identity-based array field state

[LouisHaftmann]

Summary

• replace the Vue reactivity core with the Solid signals store and asynchronous batched writes
• preserve object-array field identity across reorders and mutations while keeping primitive arrays positional
• rewrite the React and Vue adapters around the new subscription bridge
• add writable form data, setData, flush, keyed source reconciliation, and regression coverage for reset and validation semantics

Verification

• pnpm test (59 tests)
• pnpm type-check
• pnpm build
• pnpm lint:ci
• standards and spec reviews: no remaining findings

Closes #4