Add PostgreSQL DurableStore substrate for WP-03

Introduce remora-durable with pool/migrations, a RemoraApiMutation-shaped
ledger, claim-fenced outbox projection, CAS helpers, and keyset cursors,
plus a kind Job acceptance suite against the hermetic fixture.

tests/kind/durable-job.yaml.in

@@ -0,0 +1,26 @@
+apiVersion: batch/v1
+kind: Job
+metadata:
+  name: durable-substrate
+  namespace: remora-testbed
+spec:
+  activeDeadlineSeconds: 600
+  backoffLimit: 0
+  ttlSecondsAfterFinished: 300
+  template:
+    spec:
+      automountServiceAccountToken: false
+      enableServiceLinks: false
+      restartPolicy: Never
+      containers:
+        - name: tests
+          image: @@TEST_IMAGE@@
+          imagePullPolicy: Never
+          env:
+            - name: DATABASE_URL
+              value: postgres://remora:synthetic-postgres@postgres:5432/remora?sslmode=disable
+            - name: REMORA_DURABLE_REQUIRE_DB
+              value: "1"
+          resources:
+            requests: {cpu: 250m, memory: 256Mi}
+            limits: {cpu: "2", memory: 1Gi}