feat: add Reachability Center and Why Drawer components with tests

- Implemented ReachabilityCenterComponent for displaying asset reachability status with summary and filtering options.
- Added ReachabilityWhyDrawerComponent to show detailed reachability evidence and call paths.
- Created unit tests for both components to ensure functionality and correctness.
- Updated accessibility test results for the new components.

docs/api/gateway/exception-events.md

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+# Exception Events Gateway Contract (draft v0.1)
+
+Scope: stream exception workflow events (`exception.*`) for Console activity feeds and Notify integrations.
+
+## Security / headers
+- `Authorization: Bearer <token>` (or `DPoP` where configured)
+- `X-StellaOps-Tenant: <tenantId>` (required)
+- `X-Stella-Project: <projectId>` (optional)
+- Scopes: `exception:read`
+
+## Endpoint
+- `GET /exceptions/events` — Server-Sent Events (SSE) stream.
+  - Query params: `tenant`, `traceId`, `projectId` (optional)
+  - Response: `text/event-stream`
+
+## Event types
+- `exception.created`
+- `exception.updated`
+- `exception.status_changed`
+- `exception.deleted`
+
+## Rate limits (proposal)
+- Max 1 active SSE connection per browser session.
+- Heartbeat every 30s; server closes idle connections after 60s without reads.
+- When rate limited: `429` with `Retry-After`.
+
+## Notify integration
+Gateways SHOULD forward these events to Notify where configured:
+- `exception.status_changed` → `notify.event` with severity derived from exception severity and status.
+- All events include `traceId` for audit correlation.
+
+## Samples
+- `docs/api/gateway/samples/exception-events.ndjson`
+