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>

src/typechecker.rs

@@ -1229,7 +1229,7 @@ impl TypeChecker {
 
     fn check_function(&mut self, func: &FunctionDecl) {
         // Validate that all declared effects exist
-        let builtin_effects = ["Console", "Fail", "State", "Reader", "Random", "Time", "File", "Process", "Http", "HttpServer", "Test", "Sql"];
+        let builtin_effects = ["Console", "Fail", "State", "Reader", "Random", "Time", "File", "Process", "Http", "HttpServer", "Test", "Sql", "Postgres"];
         for effect in &func.effects {
             let is_builtin = builtin_effects.contains(&effect.name.as_str());
             let is_defined = self.env.lookup_effect(&effect.name).is_some();
@@ -2010,7 +2010,7 @@ impl TypeChecker {
         }
 
         // Built-in effects are always available
-        let builtin_effects = ["Console", "Fail", "State", "Reader", "Random", "Time", "File", "Process", "Http", "HttpServer", "Test", "Sql"];
+        let builtin_effects = ["Console", "Fail", "State", "Reader", "Random", "Time", "File", "Process", "Http", "HttpServer", "Test", "Sql", "Postgres"];
         let is_builtin = builtin_effects.contains(&effect.name.as_str());
 
         // Track this effect for inference