feat(ai-vue,ai-solid,ai-svelte): bring structured-output parity from ai-react PR #577 landed the typed per-message `StructuredOutputPart` design in `@tanstack/ai-react` but left the other framework packages on the old hook-level singleton state + RUN_STARTED reset. Bringing all four hook surfaces back to parity so users on Vue, Solid, and Svelte get the same multi-turn structured-output story (history-preserving typed parts on every assistant message, schema generic flowing through `messages` to `parts[i].data`) the React docs and skill already advertise. Per-package mirror of the React work: - `BaseUseChatReturn` (Vue, Solid) and `BaseCreateChatReturn` (Svelte) take the new generic. `UseChatReturn` / `CreateChatReturn` substitute `TData = InferSchemaType` when a schema is supplied, so `messages` is typed as `Array>` and `messages[i].parts.find(p => p.type === 'structured-output').data` resolves to `T` (not `unknown`) — same end-user ergonomics as React. - `setMessages` / `append` parameter types thread `TData` through as well so the public surface is consistent. `use-chat.ts` (Vue), `use-chat.ts` (Solid), and `create-chat.svelte.ts` (Svelte): - Dropped the `parsePartialJSON` import, the `rawJson` accumulator, and the `onChunk`-based `RUN_STARTED → reset → TEXT_MESSAGE_CONTENT → parse → structured-output.complete → snap` state machine. None of it is needed anymore — the per-message `StructuredOutputPart` on `messages` already carries everything. - Added `activeStructuredPart` derivations using each framework's primitive: `computed()` (Vue), `createMemo()` (Solid), `$derived.by()` (Svelte). All three implement the same `messages` scan as React's `use-chat.ts`: walk backwards to the latest user message, then scan forward for an assistant message carrying a structured-output part. Return null when no user message exists yet so a stale `final` from `initialMessages` can't leak in on first render (the SFH-6 fix from PR #577). - `partial` and `final` are now `computed` / `createMemo` / `$derived` reading from `activeStructuredPart` — `partial = part.partial ?? part.data ?? {}`, `final = part.status === 'complete' ? part.data : null`. Identical semantics across all four frameworks. Type-level tests in each package's `tests/use-chat-types.test.ts` / `tests/create-chat-types.test.ts` extended with two new assertions: - When `outputSchema` is supplied, the structured-output variant of `messages[i].parts[j]` resolves to `StructuredOutputPart` (not the default ``), and `data` types as `Person | undefined`. Locks in the schema-generic flow at the type level. - Without `outputSchema`, the variant defaults to `StructuredOutputPart` and `data` is `unknown | undefined`. Locks in the backward-compatible default. `@tanstack/ai-preact` deliberately left alone — it doesn't support `outputSchema` at all (verified by grep), which matches the docs' "useChat (React, Vue, Solid) and createChat (Svelte) all accept the same outputSchema option" claim. Adding it to preact is a separate feature, not parity work. Verification: - All three packages build cleanly (`nx run-many -t build`). - Type-level tests pass and now exercise the schema-generic flow. - `nx run-many -t test:lib,test:types,test:eslint` clean across the monorepo. 256 tests across the three frameworks (94 + 58 + 104), all green. End-to-end coverage is currently React-only (the e2e harness ships a React app at `testing/e2e/src/routes/`) — adding parallel Vue / Solid / Svelte harness apps is a separate, larger lift and out of scope for this parity pass. The unit-level type tests pin the API contract; runtime behavior is identical to React because all four hooks ride the same `ChatClient` + `StreamProcessor` underneath.