feat: add PostgreSQL driver with Postgres effect

Implements full PostgreSQL support through the Postgres effect:
- connect(connStr): Connect to PostgreSQL database
- close(conn): Close connection
- execute(conn, sql): Execute INSERT/UPDATE/DELETE, return affected rows
- query(conn, sql): Execute SELECT, return all rows as records
- queryOne(conn, sql): Execute SELECT, return first row as Option
- beginTx(conn): Start transaction
- commit(conn): Commit transaction
- rollback(conn): Rollback transaction

Includes:
- Connection tracking with connection IDs
- Row mapping to Lux records with field access
- Transaction support
- Example: examples/postgres_demo.lux
- Documentation in docs/guide/11-databases.md

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Cargo.toml

@@ -15,6 +15,8 @@ serde_json = "1"
 rand = "0.8"
 reqwest = { version = "0.11", features = ["blocking", "json"] }
 tiny_http = "0.12"
+rusqlite = { version = "0.31", features = ["bundled"] }
+postgres = "0.19"
 
 
 [dev-dependencies]