NilPy: str class field registered as tyString = inline string[N] semantics (garbage reads)

Pre-existing: NilPy registered str class fields as tyString with an 8-byte slot, but the shared AN_ASSIGN store path treats a tyString FIELD as an inline string[N] shortstring (capacity-clamped CONTENT copy via RecFieldStrCap). Result: correct length, garbage bytes — self.a = a then print(obj.a) printed spaces. Both init and dataclass paths affected.

Fix: NilPy str fields register as tyAnsiString (pointer semantics, managed); the existing tyString->tyAnsiString conversions cover ctor args and stores.