Problem

The gateway is pull-only — providers are polled and the merged view is rebuilt on a cadence. A diagnostic client expects live push: ros2_medkit exposes a trigger subsystem and an SSE stream at GET /api/v1/triggers/events. A drop-in client typically creates a trigger, then listens, so the subscription path must exist or the client receives nothing.

Proposal

Add the v1 slice of triggers + SSE to the gateway:

• A refresh-and-diff loop on the off-path tokio runtime: re-poll/merge on the cadence, diff successive MergedViews, and emit change events health_changed, fault_raised, fault_cleared.
• POST /api/v1/triggers + GET /api/v1/triggers (+ GET/DELETE /triggers/{id}): basic subscription with a minimal filter (by entity and/or severity). Rich condition predicates are explicitly deferred.
• GET /api/v1/triggers/events: an SSE stream delivering events matching the caller's triggers, framed per the captured contract (diff against the contract/golden SSE sample).

Acceptance

• POST /triggers registers a basic entity/severity subscription and GET /triggers lists it; DELETE /triggers/{id} removes it.
• GET /triggers/events streams SSE frames; an integration test that raises and clears a fault (via a provider) observes fault_raised then fault_cleared frames matching the contract shape.
• Health transitions emit health_changed.
• The refresh-and-diff loop runs off the control path (tokio side); rich predicates are documented as deferred. REQ_0900/TEST_0900 linked.

Blocked by

• medkit-gateway-axum: walking skeleton serving the read-diagnostic core over the mock provider (501 for deferred families) #81 (walking skeleton: gateway, merged view, and route surface)