The After Effects stagger idiom
id = thisProperty.propertyGroup(2).propertyIndex;
failed with "not a callable function": the expression object model had
no propertyGroup() and property groups had no propertyIndex.
- Property.propertyGroup(countUp = 1) and PropertyGroup.propertyGroup()
return the group countUp levels up; from a top-level group they reach
the layer. A count below 1, not a number, or above the layer is an
error naming propertyGroup, never a crash.
- PropertyGroup.propertyIndex: the group's 1-based index among its
siblings (Child requests now carry it).
- One new host request (Ancestor) walks up from the node's full uid
path, so it also works for the property being evaluated.
- Tests for the stagger expression, effect parameters, reaching the
layer, other layers' properties and groups and out-of-range counts;
the fuzzer seeds and tokens include the new members.
