RFC: Unified cancellation via AbortSignal across React async APIs

[RichLewis007]

React now spans client and server with transitions, Suspense/use, and server actions, but there is no unified way to cancel stale async work. This RFC proposes an opt-in, web-standard cancellation model using AbortSignal that flows consistently through these APIs, with signal.reason for diagnostics and full backwards compatibility.

The goal is to stop irrelevant work predictably, reduce wasted I/O and compute, and align React with platform semantics. The proposal includes complete examples, signal composition helpers, and integration patterns for library and framework authors.

This RFC is ready for review. I welcome design feedback and real-world use cases from the React community.