Restructure solution layout by module

src/Scheduler/__Libraries/StellaOps.Scheduler.Queue/README.md

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+# Scheduler Queue — Sprint 16 Coordination Notes
+
+Queue work now has concrete contracts from `StellaOps.Scheduler.Models`:
+
+* Planner inputs reference `Schedule` and `ImpactSet` samples (`samples/api/scheduler/`).
+* Runner segment payloads should carry `runId`, `scheduleId?`, `tenantId`, and the impacted digest list (mirrors `Run.Deltas`).
+* Notify fanout relies on the `DeltaSummary` shape already emitted by the model layer.
+
+## Action items for SCHED-QUEUE-16-401..403
+
+1. Reference `StellaOps.Scheduler.Models` so adapters can serialise `Run`/`DeltaSummary` without bespoke DTOs.
+2. Use the canonical serializer for queue messages to keep ordering consistent with API payloads.
+3. Coverage: add fixture-driven tests that enqueue the sample payloads, then dequeue and re-serialise to verify byte-for-byte stability.
+4. Expose queue depth/lease metrics with the identifiers provided by the models (`Run.Id`, `Schedule.Id`).
+
+## JetStream failover notes
+
+- `SchedulerQueueOptions.Kind = "nats"` will spin up `NatsSchedulerPlannerQueue` / `NatsSchedulerRunnerQueue` instances backed by JetStream.
+- `SchedulerQueueHealthCheck` pings both planner and runner transports; register via `AddSchedulerQueueHealthCheck()` to surface in `/healthz`.
+- Planner defaults:
+  ```yaml
+  scheduler:
+    queue:
+      kind: nats
+      deadLetterEnabled: true
+      nats:
+        url: "nats://nats:4222"
+        planner:
+          stream: SCHEDULER_PLANNER
+          subject: scheduler.planner
+          durableConsumer: scheduler-planners
+          deadLetterStream: SCHEDULER_PLANNER_DEAD
+          deadLetterSubject: scheduler.planner.dead
+        runner:
+          stream: SCHEDULER_RUNNER
+          subject: scheduler.runner
+          durableConsumer: scheduler-runners
+      redis:
+        deadLetterStream: scheduler:planner:dead
+        idempotencyKeyPrefix: scheduler:planner:idemp:
+  ```
+- Retry / dead-letter semantics mirror the Redis adapter: attempts beyond `MaxDeliveryAttempts` are shipped to the configured dead-letter stream with headers describing `runId`, `scheduleId`, and failure reasons. Set `deadLetterEnabled: false` to drop exhausted messages instead.
+- Depth metrics surface through `scheduler_queue_depth{transport,queue}`; both transports publish lightweight counters to drive alerting dashboards.
+
+These notes unblock the queue guild now that SCHED-MODELS-16-102 is complete.