Adds inline pipeline stages between the bus and the severity router:
- Inhibitor: suppresses lower-or-equal severity events when a key
in event.inhibit_keys is already active. TTL configurable, default
30 minutes.
- Grouper: coalesces events sharing group_key within a time window
(default 60s). Most recent event wins. tick() and flush_all()
drive emission; no background timers in Phase 2.2.
- build_pipeline now wires: bus -> inhibitor -> grouper -> severity_router
Phase 2.1 dispatcher tests continue to pass unchanged.
