Binder: public describe_member(kotlin_type_name, member_name) for resolvers #54

Gap in #34's contract, found by pythonx-compose: a member resolver must return (kotlin_name, keyword_map), but to build {"padding_values": "paddingValues"} it needs the member's parameter names, and there is no public path from a receiver type to a member's declaration rows (describe(module, name) needs the declaring package, which for extensions differs from the receiver's; _Hybrid._fn is private).

Decided: add python_multiplatform.describe_member(kotlin_type_name, kotlin_member_name) returning the same tuple-of-dict shape as describe(), covering members and extensions served on that type (with supertype lookup), without invoking anything. Resolver signature unchanged.

Completion: describe_member('androidx.compose.ui.Modifier', 'padding') lists the padding overloads with parameter names; unknown member -> AttributeError; tamper check; documented in KotlinSurface.kt.