Cancel idle RPC readable streams when their consumers stop

[mmkal]

Cancelling a remote readable stream now cancels its producer while it is idle. Previously the producer only learned about cancellation when it sent another chunk, leaving SSE producers running indefinitely.

Use the existing rejection message to notify the sender, and pump with an owned reader so cancellation reaches the source. In workerd 1.20260205.0, an idle pipeTo did not cancel its source after either destination failure or an AbortSignal. The explicit pump preserves write backpressure, forwards source errors, and releases locks without awaiting user cancellation callbacks. Disposed imports also release their failure callbacks instead of retaining them until the session closes.

Validation: 612 Node/workerd tests, build and type tests passed. Callback-returned consumed/locked Response cases pass on both runtimes; existing rejection behavior is preserved. The idle-cancellation and session-disconnect regressions failed before their fixes and passes with no leaked RPC imports/exports.

Review focus: stream cancellation and cleanup in src/rpc.ts, followed by writer failure signaling in src/streams.ts. No protocol message types added. This PR requests a pkg.pr.new preview for downstream integration testing; no npm release.

Codex session: 01a07b03-100a-7803-ba3f-caed48046317

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/iterate/capnweb/@iterate-com/capnweb@8

commit: 677465a

[mmkal]

I'm so sorry! An over-eager clanker just created this for me, I can't vouch for it. I will scold/update my AGENTS.md.