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/src/lib.rs

@@ -0,0 +1,29 @@
+//! PostgreSQL DurableStore substrate.
+//!
+//! Provides the pool, migration runner, transaction helpers, generic mutation
+//! ledger refining `RemoraApiMutation`, durable outbox claim/projection, and
+//! integrity-protected keyset cursor primitives. Feature packages own their
+//! domain schema additions.
+
+mod cas;
+mod cursor;
+mod error;
+mod ledger;
+mod migrate;
+mod outbox;
+mod pool;
+mod tx;
+
+pub use cas::{cas_set_text, cas_set_text_pool};
+pub use cursor::{CursorCodec, KeysetCursor, KeysetPosition, first_page_cursor, next_page_cursor};
+pub use error::DurableError;
+pub use ledger::{
+    MutationDisposition, MutationOutcome, MutationStatus, execute_mutation, execute_mutation_in_tx,
+};
+pub use migrate::{migrate, migrator};
+pub use outbox::{
+    OutboxClaim, OutboxClaimFilter, OutboxMessage, claim_batch, claim_batch_in_tx, enqueue,
+    enqueue_after, mark_projected, mark_projected_pool, release_with_error, unprojected_count,
+};
+pub use pool::{PoolConfig, connect};
+pub use tx::{BoxFuture, PgTx, advisory_xact_lock, set_lock_timeout, with_transaction};