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/cas.rs

@@ -0,0 +1,90 @@
+use sqlx::{AssertSqlSafe, PgPool};
+
+use crate::error::DurableError;
+use crate::tx::{self, PgTx};
+
+/// Optimistic compare-and-swap on a caller-owned table.
+///
+/// The table must expose `revision BIGINT NOT NULL` and a primary key column
+/// bound as `$1`. Feature packages own the surrounding schema; this helper only
+/// serializes the revision bump. Identifiers are restricted to lowercase SQL
+/// identifiers before interpolation.
+pub async fn cas_set_text(
+    tx: &mut PgTx<'_>,
+    table: &str,
+    id_column: &str,
+    id: &str,
+    value_column: &str,
+    value: &str,
+    expected_revision: i64,
+) -> Result<i64, DurableError> {
+    validate_ident(table)?;
+    validate_ident(id_column)?;
+    validate_ident(value_column)?;
+
+    let sql = format!(
+        "UPDATE {table}
+         SET {value_column} = $2,
+             revision = revision + 1
+         WHERE {id_column} = $1
+           AND revision = $3
+         RETURNING revision"
+    );
+    let next: Option<(i64,)> = sqlx::query_as(AssertSqlSafe(sql))
+        .bind(id)
+        .bind(value)
+        .bind(expected_revision)
+        .fetch_optional(tx.as_mut())
+        .await?;
+    next.map(|(revision,)| revision)
+        .ok_or(DurableError::CasConflict)
+}
+
+pub async fn cas_set_text_pool(
+    pool: &PgPool,
+    table: &str,
+    id_column: &str,
+    id: &str,
+    value_column: &str,
+    value: &str,
+    expected_revision: i64,
+) -> Result<i64, DurableError> {
+    let table = table.to_owned();
+    let id_column = id_column.to_owned();
+    let id = id.to_owned();
+    let value_column = value_column.to_owned();
+    let value = value.to_owned();
+    tx::with_transaction(pool, move |tx| {
+        Box::pin(async move {
+            cas_set_text(
+                tx,
+                &table,
+                &id_column,
+                &id,
+                &value_column,
+                &value,
+                expected_revision,
+            )
+            .await
+        })
+    })
+    .await
+}
+
+fn validate_ident(value: &str) -> Result<(), DurableError> {
+    if value.is_empty()
+        || value.len() > 63
+        || !value.chars().all(|character| {
+            character.is_ascii_lowercase() || character.is_ascii_digit() || character == '_'
+        })
+        || value
+            .chars()
+            .next()
+            .is_none_or(|character| !(character.is_ascii_lowercase() || character == '_'))
+    {
+        return Err(DurableError::Config(format!(
+            "invalid SQL identifier '{value}'"
+        )));
+    }
+    Ok(())
+}