[EMSUSD-1528] Support proxy shape Maya instancing

Maya instancing of proxy shape nodes is unsupported. Instancing proxy shape nodes does not draw the instanced copies in the viewport.

If this is valuable to customers, instancing of proxy shapes should be supported.

USD instancing is different and useful, but does not serve exactly the same purpose.

Code such as MayaUsdProxyShapeBase::parentTransform() does not consider the path, and simply uses the first path. StagesSubject::stageChanged() translates USD notifications to a single UFE path; if proxy shapes are instanced, we would need multiple notifications, one per path.

Start by reading MayaUsdProxyShapeBase::parentTransform() and StagesSubject::stageChanged(), focusing on how instanced proxy paths are currently reduced to one UFE path. Trace the USD notification flow and determine the required per-path handling. Done means instanced proxy shape copies draw correctly in Maya's viewport without breaking USD instancing behavior.

Written by the indexing model from the issue text.