Add handle-based Python C API for per-loop isolation (#5)

Add isolated event loops with `ErlangEventLoop(isolated=True)`

Each isolated loop has its own pending queue for full callback isolation.
Full asyncio support (timers, FD operations) via shared router.
Useful for multi-threaded Python, sub-interpreters, and free-threaded Python (3.13+).

Implementation:
- Add `isolated=False` parameter to `ErlangEventLoop.__init__`
- Add handle-based Python C API (`_loop_new`, `_run_once_native_for`, etc.)
- Add shared router for per-loop created loops (`set_shared_router` NIF)
- Timer messages include LoopRef for correct dispatch to isolated loops
- Router stores LoopRef with each timer, dispatches to correct loop

See `docs/asyncio.md` for usage and architecture details.

Author: benoitc

CHANGELOG.md

@@ -18,6 +18,12 @@
 
 ### Added
 
+- **Isolated Event Loops** - Create isolated event loops with `ErlangEventLoop(isolated=True)`
+  - Each isolated loop has its own pending queue
+  - Full asyncio support (timers, FD operations) via shared router
+  - Useful for multi-threaded Python applications
+  - See `docs/asyncio.md` for usage and architecture details
+
 - **Python Logging Integration** - Forward Python's `logging` module to Erlang's `logger`
   - `py:configure_logging/0,1` - Setup Python logging to forward to Erlang
   - `erlang.ErlangHandler` - Python logging handler that sends to Erlang