Accept trailing arguments uniformly across the apply family

[Bronek]

sum::apply/apply_r accept trailing arguments after the alternative's content — the member leg of fn::apply's engine protocol: the free function dispatches fn::apply(fn, sum, extras...) through the member, and the multi-operand fold recurses the same way. The rest of the family never got the same shape, for no design reason: apply_type/apply_type_r have no free-function counterpart, so nothing forced the arguments onto them, and choice is an atom to the engine (free apply never dispatches it), so its members stayed minimal. The internal machinery already anticipates the arguments — the type-indexed _invoke_type is variadic, and the tag prepender forwards whatever follows the tag.

Rectify additively, keeping sum::apply's shape (the engine protocol cannot narrow):

1. sum::apply_type and apply_type_r accept trailing arguments, appended after the alternative's unpacked content: arms become (tag, elements..., extras...).
2. Parity across the family: choice's apply/apply_r/apply_type/apply_type_r, and optional/expected's apply_type/apply_type_r, accept trailing arguments the same way (their untagged apply/apply_r already do). On optional, the empty arm receives (std::nullopt_t, extras...), mirroring the untagged empty arm's (extras...); expected<void, E>'s value arm likewise (std::in_place_t, extras...).

Intended for release-0.1: the family's shape should settle before the first tag.