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.

crates/remora-durable/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "remora-durable"
+edition.workspace = true
+publish = false
+rust-version.workspace = true
+version.workspace = true
+
+[dependencies]
+base64.workspace = true
+hmac.workspace = true
+remora-domain.workspace = true
+serde.workspace = true
+serde_json.workspace = true
+sha2.workspace = true
+sqlx.workspace = true
+thiserror.workspace = true
+time.workspace = true
+tokio.workspace = true
+
+[dev-dependencies]
+tokio = { workspace = true, features = ["macros", "rt-multi-thread", "sync", "time"] }