docs(p15): add G9G-3 cluster spec mini-plan

sw-block/design/v3-phase-15-g9g3-cluster-spec-yaml-mini-plan.md

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+# V3 Phase 15 G9G-3 - Cluster Spec YAML Facade Mini-Plan
+
+Date: 2026-05-03
+Status: architect draft; usability facade only
+Branch target: `p15-g9g3/cluster-spec-yaml`
+Scope: declarative bootstrap YAML that feeds existing lifecycle/product-loop stores
+
+## 0. Why This Gate Exists
+
+G9G closed the first product-loop assignment path using `--lifecycle-placement-seed`.
+That seed file is a good QA bridge but not a product-shaped operator surface.
+
+G9G-3 introduces a small declarative facade:
+
+```powershell
+blockmaster --cluster-spec m01.yaml --authority-store ... --lifecycle-store ...
+```
+
+The YAML is converted into existing lifecycle facts; it does not bypass the
+G9D/F/G9F-2/G9G chain.
+
+## 1.A Bindings
+
+1. **Facade only**: cluster spec imports lifecycle facts; it must not call publisher directly.
+2. **No new truth**: YAML fields become desired volume, node inventory, and placement intent only.
+3. **Existing pipeline**: assignment still requires product-loop verification and publisher minting.
+4. **No proto/API change**: first slice is startup bootstrap only.
+5. **Temporary simplicity**: schema can cover existing-replica placement first; blank-pool allocation remains follow-up.
+
+## 2. Minimal YAML Shape
+
+```yaml
+volumes:
+  - id: v1
+    size_bytes: 1048576
+    replication_factor: 1
+
+nodes:
+  - id: s2
+    data_addr: 127.0.0.1:9202
+    ctrl_addr: 127.0.0.1:9102
+    replicas:
+      - volume_id: v1
+        replica_id: r2
+        size_bytes: 1048576
+
+placements:
+  - volume_id: v1
+    desired_rf: 1
+    slots:
+      - server_id: s2
+        replica_id: r2
+        source: existing_replica
+```
+
+## 3. TDD Plan
+
+1. `TestClusterSpec_ImportsDesiredVolumeNodeAndPlacement`
+2. `TestParseFlags_ClusterSpecOptional`
+3. `TestG9G_L2ClusterSpecPublishesAssignmentToBlockvolume`
+
+## 4. Close Criteria
+
+1. YAML imports into lifecycle stores.
+2. Existing G9G product loop consumes imported placement.
+3. Real subprocess L2 reaches blockvolume assignment-backed `Healthy=true`.
+4. No authority publisher call is added to the import path.
+
+## 5. Non-Claims
+
+- Not a full product API.
+- Not CSI.
+- Not dynamic reconfiguration.
+- No blank-pool replica-id allocator.
+- No M01 hardware evidence.