_pick_weighted returned options[0] when every weight was zero. That looked
like a working pick - a real option, right shape, right place - and stayed
wrong silently until the list was reordered and the choice changed. It hid
both over-narrowed constraints and a wildcard whose options were all
disabled.
Now returns None, which every caller already turned into an empty binding.
That is what the null option means, so the engine had the vocabulary for
this already.
Applies to all three pools that constraints can zero: the chain handler, the
nested-@{} path in the resolver, and the SPA preview. The inline {a|b}
branch weights are left alone - a user writing {0::a|0::b} is a different
situation and constraints never touch it.
Three tests asserted the old behaviour and are rewritten to state the new
one; the constraint_excludes_all_options warning stays, and now
distinguishes an over-narrowed pool from a legitimate null roll.
