Resolve sublibrary downgrade failures behind the old #417 disable (now re-enabled)

`DowngradeSublibraries.yml` was previously **disabled** (`if: false`, "waiting on upstream compat fixes. See #417"). The CI-centralization conversion to `sublibrary-downgrade.yml@v1` re-enabled it (and now tests all `lib/*` sublibraries with no exclusions). So the downgrade-compat failures that motivated #417 will now show up red again. Next step: work through them — for each failing sublibrary, bump the offending dependency's `[compat]` lower bound to a version that resolves/precompiles under downgrade (the real fix), rather than re-disabling. See #417 for the original context. _Follow-up from the SciML CI-centralization work._