feat: Enhance MongoDB storage with event publishing and outbox support

- Added `MongoAdvisoryObservationEventPublisher` and `NatsAdvisoryObservationEventPublisher` for event publishing.
- Registered `IAdvisoryObservationEventPublisher` to choose between NATS and MongoDB based on configuration.
- Introduced `MongoAdvisoryObservationEventOutbox` for outbox pattern implementation.
- Updated service collection to include new event publishers and outbox.
- Added a new hosted service `AdvisoryObservationTransportWorker` for processing events.

feat: Update project dependencies

- Added `NATS.Client.Core` package to the project for NATS integration.

test: Add unit tests for AdvisoryLinkset normalization

- Created `AdvisoryLinksetNormalizationConfidenceTests` to validate confidence score calculations.

fix: Adjust confidence assertion in `AdvisoryObservationAggregationTests`

- Updated confidence assertion to allow a range instead of a fixed value.

test: Implement tests for AdvisoryObservationEventFactory

- Added `AdvisoryObservationEventFactoryTests` to ensure correct mapping and hashing of observation events.

chore: Configure test project for Findings Ledger

- Created `Directory.Build.props` for test project configuration.
- Added `StellaOps.Findings.Ledger.Exports.Unit.csproj` for unit tests related to findings ledger exports.

feat: Implement export contracts for findings ledger

- Defined export request and response contracts in `ExportContracts.cs`.
- Created various export item records for findings, VEX, advisories, and SBOMs.

feat: Add export functionality to Findings Ledger Web Service

- Implemented endpoints for exporting findings, VEX, advisories, and SBOMs.
- Integrated `ExportQueryService` for handling export logic and pagination.

test: Add tests for Node language analyzer phase 22

- Implemented `NodePhase22SampleLoaderTests` to validate loading of NDJSON fixtures.
- Created sample NDJSON file for testing.

chore: Set up isolated test environment for Node tests

- Added `node-isolated.runsettings` for isolated test execution.
- Created `node-tests-isolated.sh` script for running tests in isolation.

docs/modules/concelier/operations/observation-events.md

@@ -0,0 +1,31 @@
+# Observation Event Transport (advisory.observation.updated@1)
+
+Purpose: document how to emit `advisory.observation.updated@1` events via Mongo outbox with optional NATS JetStream transport.
+
+## Configuration (appsettings.yaml / config)
+```yaml
+advisoryObservationEvents:
+  enabled: false            # set true to publish beyond Mongo outbox
+  transport: "mongo"        # "mongo" (no-op publisher) or "nats"
+  natsUrl: "nats://127.0.0.1:4222"
+  subject: "concelier.advisory.observation.updated.v1"
+  deadLetterSubject: "concelier.advisory.observation.updated.dead.v1"
+  stream: "CONCELIER_OBS"
+```
+
+Defaults: disabled, transport `mongo`; subject/stream as above.
+
+## Flow
+1) Observation sink writes event to `advisory_observation_events` (idempotent on `observationHash`).
+2) Background worker dequeues unpublished rows, publishes via configured transport, then stamps `publishedAt`.
+3) If transport disabled/unavailable, outbox accumulates safely; re-enabling resumes publishing.
+
+## Operational notes
+- Ensure NATS JetStream is reachable before enabling `transport: nats` to avoid retry noise.
+- Stream is auto-created if missing with current subject; size capped at 512 KiB per message.
+- Dead-letter subject reserved; not yet wired—keep for future schema validation failures.
+- Backlog monitoring: count documents in `advisory_observation_events` with `publishedAt: null`.
+
+## Testing
+- Without NATS: leave `enabled=false`; app continues writing outbox only.
+- With NATS: run a local `nats-server -js` and set `enabled=true transport=nats`. Verify published messages on subject via `nats sub concelier.advisory.observation.updated.v1`.