A keyword after a *unpack on a run-time dispatched receiver was refused again

Found 2026-10-19 in the full row sweep. test_nilpy_a_keyword_argument_after_a_star_unpack (fixed 09-12 in d220115cf3) stopped compiling: pick(True).n(*xs, forced=9) gave "dispatched at run time, and that path cannot combine *unpacking with keyword arguments yet". Bisected to d4e33b4a83 (10-10). That commit sent star calls on an ambiguous receiver to pydyn_methl so they bind on the run-time class, and it refused a keyword beside the star there. Nobody saw it because the recipe was stopping at row 593.

Fix:

• Parser: when the keywords are a trailing run, the kwspec names only that run, marked >. Any other mix still refuses, with a message that says why.
• PyDynMethL right-aligns a > kwspec once the star's length is known.
• PyBindHostKwArgs fills a slot that a keyword skips (*xs, x=9 past a defaulted c) from the callee's signature defaults. It used to refuse this with "can only omit TRAILING parameters". A slot with no default is still refused.

Test: test_nilpy_a_dynamic_star_call_takes_trailing_keywords (CPython output).