NilPy: range for-loop with a pre-existing VARIANT loop variable corrupts it

Found while landing feature-nilpy-comprehension-if-filter. If x was earlier a container-loop target (typed tyVariant), a later for x in range(n): drove the variant slot RAW with the counted-for/while lowering: an untagged integer under a stale tag. x > 2 matched nothing (silent!), x % 2 raised "expected a number, got str". Repro: comp over a list, then plain for x in range(6): if x > 2: n += 1 → n = 0 (CPython 3). Pre-existing — reproduces on the pinned compiler.

Fix (same commit as the comprehension filter): when the range target is a variant, count with a hidden int local and box it into the variant at the top of each iteration (x = __py_i), for both the step-while and counted-for lowerings. Covered by test_nilpy_comp_filter.npy.