[skjiisa]

Why

From 0.5.7, USVFS links the VC++ runtime dynamically (0.5.6.1, shipped with MO2 2.5.2, had it built in). MO2 injects USVFS into the programs it starts, so it uses their runtime: the game folder's, or the Proton prefix's system32. A Skyrim prefix usually has the 2016 msvcp140.dll from the game's own redistributable plus Wine's built-in vcruntime140_1.dll. On a Steam Frame with ModOrganizer2/usvfs#93's 0.5.7.2 build, Skyrim and Explorer++ started from MO2 died right after inithooks called, with a jump to address 0 inside vcruntime140_1.dll. Installing Microsoft's redistributable into the prefix fixed it, and modded Skyrim then ran. #60's copy next to ModOrganizer.exe can't reach the game process. This matters on every platform once MO2 ships USVFS 0.5.7+, and on the Frame as soon as #93 lands.

What

• Detect (vcruntime.prefix_runtime_problems, usvfs_needs_prefix_runtime): only when the instance's usvfs_x64.dll is 0.5.7+, and any of msvcp140/vcruntime140/vcruntime140_1 is missing, Wine's built-in stand-in (marker in its DOS stub), or older than 14.40. On the Frame's pre-install prefix the old vcruntime140_1 reported 14.42 but was Wine's, so version alone isn't enough.
• Offer: a warning row with "Install Visual C++ runtime…" in the dashboard's MO2 card, only when needed, behind a confirmation dialog. Never automatic.
• Install (service.install_prefix_runtime): back up the runtime DLLs to data/modsync/backups/, fetch the pinned and checksummed VC_redist.x64.exe (same pin as Give MO2 its own current Visual C++ runtime before launching it #60, cached), and run /install /quiet /norestart through the game's Proton and Steam Linux Runtime via the new launch.run_in_prefix (with flatpak-spawn --host from the Flatpak). Then re-check. It refuses while MO2 runs, and failures name the backup.
• doctor: each instance's USVFS version, plus this check for 0.5.7+.

Testing

• Unit tests: detection (old, stand-in, missing, current), the USVFS version threshold, the backup selection, and the install flow (success, installer failure, still old afterwards, refusing while MO2 runs).
• uv run pytest -q: 362 passed, 18 skipped.
• On the Frame: detection flags the real pre-install system32 backup and passes the current prefix. The real install path ran through Steam's ARM64 Proton from source and from inside the aarch64 Flatpak (about 15 s each, clean re-check, backups written). doctor shows usvfs 0.5.6.1 and usvfs 0.5.7.2: game prefix has a current VC++ runtime for the two test instances.
• Not tested: clicking through the dialog in the GUI (the row is hidden because this machine's prefix is already current).

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