93b7947d7c
Replaces manual extractRefs functions with declarative schema-level
casRef annotations. Walks Zod v4 schemas recursively, collecting
string values from fields marked with .meta({ casRef: true }).
Supports: objects, arrays, discriminatedUnion, nullable/optional.
8/8 test cases pass (flat, nested, union, null, mixed).
Refs #285, addresses #286