(PDB-5739) Reduce factsets "stable||volatile" merging

[rbrw]

Without more potentially substantial changes to the query engine, we can't drop all the merging (e.g. the facts projection from the inventory endpoint) and switch to independent stable and volatile indexes, so for now, just consider some incremental changes along those lines. cf. #3955

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}