NilPy: list type v1 (pylib TPyList, literals, subscripts, methods, len)

Part of [[feature-nilpy-corpus-uforth]] milestone 1. The biggest single rung: uforth's stacks are List[Any] (Word.forth_body, Frame.body, VM.stack...).

Design (settled this session):

• TPyList = a real class in a new builtin unit compiler/builtin/pylib.pas (Track B file, language-neutral lib): growable raw-memory array of 16-byte variant slots ({VType,Payload} — builtin.pas's TVariantRecord model, VType 0 = None). Reference semantics like Python (class = heap pointer). Methods append (returns Self for literal-desugar chaining) / pop / insert / clear / count; property Items[i] default gives xs[i] read+write through the EXISTING default-indexed-property machinery — no subscript work in the frontend. Negative indices Python-style; out-of-range = runtime error halt.
• Frontend: [a, b, c] desugars to create + chained .append() calls (one expression, no statement context needed); List[...] annotations map to tyClass/TPyList; len(x) resolves to a plain pylib function; pylib is auto-used by every .npy.
• Frozen-string build assumption: variant payloads are copied raw (no refcount). The managed-string build needs a revisit — noted here.

Later rungs: slices, for-in over lists, dict (same pattern: TPyDict), in/remove/index/extend/sort.

Gate

test-nilpy green (+ test_nilpy_list.npy vs CPython), self-host, quick tier.