read_manifest(
db_path) ->manifest,err
Read and validate a LILPACK manifest from a MNEME database
Opens the MNEME database at db_path and reads the __lilpack
manifest keyspace. Validates required fields (version, name, modules,
signature, pubkey) and decodes optional JSON fields (dependencies,
builtins, modes, cli). Returns the manifest table on success, or
nil and an error message on failure. Unsigned packages are rejected.