Rebuild on stdlib loop machinery: subclass BaseEventLoop, remove Tokio, require Python 3.11+

[jph00]

The loop now subclasses base_events.BaseEventLoop and borrows the selector-loop method families it needs, so the previously transcribed create_connection/create_server/start_tls/subprocess bodies are inherited per-version instead of frozen at transcription time. SubprocessTransport is 20 lines over BaseSubprocessTransport, keeping only the executor spawn; transports adopt the stdlib _FlowControlMixin and gain BufferedProtocol support. The Tokio host is gone: the driver blocks directly in the polling reactor with the GIL released, which removes the ~1ms timer-granularity penalty, and embedding runtimes wake the loop through its thread-safe scheduling path. Rust drops to three files (243 lines); Python to ~400.

Crate API break (0.2.0): TokioCore and PyReactor::with_handle are removed. The Python surface is unchanged. Python floor rises to 3.11 (a 12-line shim covers the _run_forever_setup helpers that first appear in 3.12); CI now tests 3.11 through 3.14 and ships wheels for the same. DEV.md documents the architecture, the borrowed surface, and local multi-version testing via uv.