Look up an estimator in the internal registry and return a descriptor containing metadata and the estimator function.
Value
A list with elements:
estimator_id: character scalar
type: character scalar
generator: function(df, config, tau, ...) returning an estimator result
oracle: logical, TRUE if this is an oracle estimator
oracle_columns: character vector of eligible oracle columns
oracle_default_columns: character vector of default-granted oracle columns
supports_qst: logical, TRUE if estimator returns QST
version: character scalar
description: character scalar
source: character scalar indicating origin (core/optional/external)
requires_pkgs: character vector of required packages