NilPy: @dataclass v1 (decorator syntax, annotated scalar fields, defaults)

Part of [[feature-nilpy-corpus-uforth]] milestone 1.

• Lexer: @ -> tkAt.
• Module level: @dataclass (with optional ()) before class; any other decorator = clean error. Class-level name: type [= literal] lines declare fields (int/float/bool/str + class-typed); a synthesized create ctor takes one positional param per field; omitted trailing args fill from literal defaults at the call site (kwargs later).
• Class-typed annotations (p: Point) now accepted in def/method params and return types, with RecName wired for field access.
• str fields register as tyAnsiString (see bug-nilpy-string-class-field).

Fields with container types (List/Dict/Optional/...) still error — next rung is the list/dict design. field(default_factory=...) not supported.

Regression: test/test_nilpy_dataclass.npy in make test-nilpy.