Two attribute target spellings are refused

Found 2026-09-24 (frankb-12) varying the receiver shapes of the unpack/chain store group. Measured at HEAD against CPython 3:

def forloop(b):
    for b.s in range(3):      # pxx: expected 'in' before '.'
        pass
    print(b.s)                # CPython: 2

def inlist(xs):
    xs[0].s, xs[1].s = 5, 6   # pxx: expected '=' before ','
    print(xs[0].s, xs[1].s)   # CPython: 5 6

Loud refusals, so compat rather than a bug: nothing compiles wrong. Ranked on how much real code uses them, which is little: measured 2026-09-24, ZERO occurrences of either across 195 .py files (/home/neo/lekkerzeilen and /home/neo/projects/uforth, line-grep for for <name>.<attr> and for <name>[...].<attr> , ... =; a multi-line target list would escape the grep). The unpack target parser already handles name.attr and name[...]; the second shape is the composition of the two. The for-target parser takes names and nested groups only.