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/lib.sh

@@ -276,7 +276,8 @@ destroy_fixture() {
     "$RUNTIME/dependencies.yaml" \
     "$RUNTIME/fixture.env" \
     "$RUNTIME/probe.yaml" \
-    "$RUNTIME/sample-job.yaml"
+    "$RUNTIME/sample-job.yaml" \
+    "$RUNTIME/durable-job.yaml"
 }
 
 kube() {