expose main-loop file descriptor hooks

docs/plugin-api-additions/fd.md

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+# Python descriptor hooks
+
+`hook_fd(fd, callback, userdata=None, flags=FD_READ)` returns an ordinary hook
+handle accepted by `unhook()`. Callback arguments are `(fd, ready_flags, userdata)`;
+return True to retain the watch and False/None to remove it. Exceptions print a
+traceback and remove the watch. Unloading the script removes its watches.
+The callback runs in the context captured during registration, and that context
+is restored afterwards. A closed context removes the watch.
+
+Use nonblocking IO and bounded reads/writes. The descriptor belongs to the script;
+unhook before closing it. On Windows socket descriptors use the default flags;
+CRT descriptors need FD_NOTSOCKET. The existing C API uses int descriptors: values
+outside its range are rejected rather than truncated. This does not add threads.
+
+Example:
+```python
+watch = zoitechat.hook_fd(sock.fileno(), on_readable, flags=zoitechat.FD_READ)
+# def on_readable(fd, flags, userdata): ...; return True
+```