Consolidate reactor backends into parameterized templates

Replace 42 near-identical per-backend files (14 each for epoll, kqueue,
select) with shared parameterized templates and per-backend traits +
types files. Each reactor backend drops from 14 files to 3 (scheduler,
traits, types). Net: +2,624 / -7,980 lines across 64 files.

Per-backend traits structs capture all platform-specific behavior
(socket creation flags, write/accept policies, fd configuration,
SO_LINGER hooks, descriptor_state) in one location per backend. Shared
templates (reactor_socket_finals, reactor_service_finals, reactor_stream
_ops, reactor_datagram_ops, reactor_backend) provide a single
implementation of socket/service/acceptor/op logic that all three
backends instantiate via per-backend types files.

Author: mvandeberg