set methods missing: issubset/issuperset/union/intersection/difference/discard

Found by proactive CPython-diff sweeping, right after landing the set OPERATOR forms (&/|/-/^, see bug-nilpy-set-and-dict-operators-do-raw-pointer-arithmetic) — the equivalent NAMED methods (s.union(t) etc.) didn't exist at all (TPyList has no method issubset). .add() already existed (the dedup insert every set literal uses) and .remove() already existed (shared with list.remove), but the rest of the set method surface was missing.

Fix

Added ordinary TPyList instance methods (compiler/builtin/pylib.pas) — no parser wiring needed, since these resolve through the normal class-method-call path like any other method:

• issubset/issuperset — linear scan via the existing pycontains membership helper.
• union/intersection/difference — thin wrappers over the operator implementations already added for |/&/- (pyset_or/pyset_and/ pyset_sub).
• discard — like remove() but does not raise when the value is absent (checks pycontains first).

Verified against CPython (values only — a set still displays with list brackets, the separate tracked cosmetic gap, so the regression test uses sorted(...) around the container-returning methods to check computed values without depending on that unrelated display difference). Regression test test/test_nilpy_set_methods.npy gated in test-nilpy. Self-host confirmed byte-identical (pylib.pas is a runtime unit, no compiler rebuild needed, but make pxx-debug run anyway per this session's habit).

Log

• 2026-07-31 — resolved, commit HEAD.