WrappedSiteCalculator: Consider type stability and GPU memory barriers

[jameskermode]

From PR #313 review (non-blocking comment by @cortner):

The WrappedSiteCalculator struct has some design considerations for future work:

1. co_ps field: Currently not supported, but left as placeholder for committee parameters
2. GPU memory barriers: Need to discuss how to best implement without breaking GPU memory barriers
3. Type stability: The co_ps::Any field breaks type stability. Questions to consider:
   • How important is type stability for exporting?
   • If unimportant, model, ps, st could all be Any for more flexibility (e.g., allow updating st with preallocated memory)
   • Calculator then acts as a function barrier to keep type instability at the outermost layer

Reference: #313 (comment)