ch_func_to_all_pk: unify the excess-keyword policy of the with-*args and without-*args paths

[thorwhalen]

Found while landing #92 and #93 (adversarial review of the fix-variadics-all-pk branch). Pre-existing, not introduced by either.

ch_func_to_all_pk has two policies for excess keyword arguments, depending on whether the function has *args:

from i2.signatures import ch_func_to_all_pk, tuple_the_args

def f4(a, b=2, **kwargs): return a, b, kwargs
def g(a, *args, **kwargs): return a, args, kwargs

ch_func_to_all_pk(f4)(1, x=3, q=4)              # (1, 2, {})                      extras DROPPED, though f4 has **kwargs
ch_func_to_all_pk(f4)(1, kwargs={'x': 3}, q=4)  # (1, 2, {'x': 3})                q dropped
ch_func_to_all_pk(g)(1, (2,), x=3, q=4)         # (1, (2,), {'x': 3, 'q': 4})     extras FORWARDED
tuple_the_args(g)(1, (2,), x=3, q=4)            # (1, (2,), {'x': 3, 'q': 4})     forwarded
ch_func_to_all_pk(f4)(1, 2, 3)                  # TypeError: dict() argument after ** must be a mapping, not int

The no-*args path keeps the 0.1.74 behaviour because meshed's hook_up calls it with a whole scope dict and relies on extras being dropped (#92 pinned that in a test). The *args path (#92) forwards extras. The last line is a sharp edge of the no-*args path: a third positional lands on the kwargs slot.

Both policies live inside ch_func_to_all_pk's wrapper, and the advertised signature would not change, so unifying them is a one-function edit that does not touch meshed / py2http / FlexFuncFanout: route the no-*args path through all_pk_sig.map_arguments(allow_excess=True) + _args_and_kwargs_from_all_pk_arguments against func_sig too, and pick one rule for extras (drop them, or forward them when the function has **kwargs). The decision is which rule; meshed's reliance on "drop" is the constraint.