Fixes #15275

Problem

In RSC Framework Mode, an aborted/cancelled document request can crash the production server:

TypeError: Invalid state: Unable to enqueue
    at flushBufferedChunks (...)
    at Timeout._onTimeout (...)

injectRSCPayload buffers HTML chunks and flushes them in a setTimeout(..., 0) callback. When the client aborts, the readable side of the TransformStream is cancelled while that flush is still pending — the timer then calls controller.enqueue() on a cancelled stream. Because the timer callback is async, the throw becomes an unhandled rejection, which exits the Node process. The transformer had no cancel() hook, so the pending timer and the RSC payload stream were never cleaned up.

Fix

• Add a cancel(reason) handler to the transformer: it flags cancellation, clears the pending flush timer, drops the buffered chunks, cancels the underlying RSC payload stream (via its reader when streaming has started), and resolves the flight-data promise.
• Guard the flush timer callback and the writeRSCStream write loop so late timers / in-flight reads become no-ops after cancellation instead of enqueueing into a cancelled stream.

The transformer is typed with a small extension (Transformer & { cancel? }) since the repo's current TS lib doesn't yet include the standard Transformer#cancel member.

Tests

Added packages/react-router/__tests__/rsc/html-stream-test.ts with two regression tests, covering an abort while a flush is pending (the reported crash) and an abort mid-RSC-stream. Both assert no unhandled rejection escapes a timer and that the RSC payload stream gets cancelled.

• Without the fix: both fail (the flush-pending case reproduces the exact Invalid state: Unable to enqueue rejection).
• With the fix: both pass. Typecheck, ESLint and Prettier are clean. (Note: the sibling rsc/server-test.ts suite currently fails to load on a clean main checkout in the same way, unrelated to this change.)

A change file is included (patch.rsc-html-stream-cancel.md), and I've signed the CLA in contributors.yml as part of this PR.