IronTide engine runtime: the per-torrent actor, the per-peer I/O loops, and
the non-leaf engine infrastructure (disk I/O, tracker management, alerts,
streaming, extensions, SSL).
Renamed from the ROADMAP's tentative irontide-torrent-actor at the M244b
eng-review (Option 3 hybrid + rename). Sits above the pure leaves
(irontide-engine-support, irontide-session-types, irontide-peer-io,
irontide-settings) and below session-core (irontide-session), which
consumes this crate's public surface via irontide_engine::….