* fix: accept scalar or array primary in api_plugin_db_table_create()
api_plugin_db_table_create() inline-interpolated $data['primary'] as a string,
so a plugin passing an array primary produced PRIMARY KEY (`Array`) and table
creation failed on a fresh install. Route the primary through
db_format_index_create() (already used for the keys just below), which accepts
either a scalar column name (the legacy string form, required for backward
compatibility) or an array of columns -- matching db_table_create() and
db_update_table().
* tests: cover scalar and array/composite primary DDL in api_plugin_db_table_create()
TheWitness