Constant SELECT_EXEC_EXISTS_SQL 

pub const SELECT_EXEC_EXISTS_SQL: &str = "SELECT 1 FROM ff_exec_core WHERE partition_key = ?1 AND execution_id = ?2";

Existence probe — matches PG reference’s EXISTS exec_core pre-check so a non-existent execution surfaces NotFound rather than an empty page. Binds: ?1 partition_key, ?2 execution_id BLOB.