splash-only branch: strip the MCM to a save-preserving splash stub for FSMP 4.x

## Need
FSMP 4.0 removed "FSMPM - The FSMP MCM.esp" from the installer (configuration moved to the SKSE Menu Framework menu). Skyrim damages a save-game when an esp it references disappears, so players updating mid-playthrough from FSMP 3.x need a way to keep the esp alive.

## Solution
A long-lived `splash-only` branch that strips the MCM down to a stub:
- The menu shows only the splash page with the FSMP logo and the text "Use SKSE Menu Framework for the new menu".
- All configuration machinery is removed: no JContainers/PapyrusUtil/ConsoleUtil requirements at runtime, no configs.xml / configsPresets shipped.
- On saves updating from the full MCM, the stub releases the JContainers map the full MCM retained in the co-save.
- The esp itself is unchanged, so existing saves keep their plugin reference.

The hdtSMP64 continuous integration will fetch this branch (and only this branch) to package the stub as an optional FOMOD install (see the matching hdtSMP64 issue).

Start by comparing the full MCM with the requested splash-only branch and identify the existing splash page, configuration machinery, shipped configs, and co-save map handling. Verify that the stub keeps the esp unchanged, displays only the specified splash text, releases the retained JContainers map, and has no runtime configuration dependencies. Then check the matching hdtSMP64 issue for the branch and optional FOMOD packaging expectations.

Written by the indexing model from the issue text.