Add cooperative cancellation and refine scheduler capabilities.

[samuel-williams-shopify]

Add runtime-independent Cancellation, Cancelled, and defer_cancel so futures can respond to shutdown requests and continue asynchronous cleanup. Rename the portable scheduler capabilities from FileIo to File and from Network to Socket across the facade, native selectors, and Tokio adapter.

Cancellation clones share persistent state, child signals receive ancestor cancellation, and dropping a signal does not request shutdown. defer_cancel invokes a synchronous callback once and continues polling the protected future. Tests cover registration races, wakeups, ancestor lifetimes, deep trees, callback timing, pinning, and cleanup on Socketry and Tokio. Existing task and scheduler destruction semantics remain in place; cooperative runtime draining and cancellation-aware I/O are future work.

The executor design records the proposed scheduler interface, capability boundaries, prepared linking, owned and registered buffers, and pool receives, with examples for file reads, forwarding, synchronization, and service shutdown. with_previous_buffer remains hypothetical with no selected implementation target. Update examples, package documentation, release notes, and the architectural gaps alongside these changes.