feat: streaming RPC results (return a ReadableStream/AsyncIterable from an action)

[vivek7405]

Sub-issue of #488. webjs's RPC is request/response only. An action cannot return a ReadableStream / AsyncIterable and have it stream to the client (AI token streams, progress, large result sets). TanStack Start supports this via a multiplexed frame protocol (packages/start-server-core/src/frame-protocol.ts) + a raw-response marker (x-tss-raw) + late stream registration, serialized with seroval's toCrossJSONStream.

Design / approach

When an action (any verb, typically GET or POST) returns a ReadableStream / AsyncIterable / async generator, stream it over the RPC wire instead of buffering. The client stub returns an async iterable / stream the component consumes. Reuse webjs's existing streaming infra where possible (<webjs-stream> / Suspense streaming are render-side; this is RPC-result-side). Consider a framed wire (length-prefixed chunks) so a stream can interleave with the serialized envelope. Compose with #472 seeding (a streamed result is not seeded) and the verb work in #488.

Acceptance criteria

• An action returning a ReadableStream / AsyncIterable streams over the RPC; the client stub yields chunks as they arrive.
• Back-pressure respected; the stream cancels when the client disconnects (ties into the AbortSignal sub-issue).
• Rich values inside the stream round-trip through webjs's serializer.
• Tests: unit (framed encode/decode), integration (createRequestHandler streams), browser/e2e (a token stream renders incrementally).
• Docs updated (agent-docs/advanced.md, data-fetching page).