Detect the installed KSA version from KSA.dll's file version

[Maximilian-Nesslauer]

Problem

KsaBuildVersionProvider detects the installed version by reading the newest Content/Versions/*.json and its build field. That works, but relies on file timestamps ("newest by LastWriteTimeUtc") and on the Versions folder being intact.
KSA.dll's PE file version (FileVersionInfo.GetVersionInfo(<GameDir>/KSA.dll).FileVersion) carries the exact same 4-part version (e.g. 2026.6.9.4750) and can be read without loading the assembly, similar to how the KSP2 provider falls back to FileVersionInfo of the exe.

Tasks

• Try KSA.dll's FileVersion first in KsaBuildVersionProvider, keeping the Content/Versions scan as fallback.
• Keep the fallback authoritative for faked instances: GameInstanceManager.FakeInstance writes a minimal Content/Versions file (Make instance fake work for KSA #34) and creates no KSA.dll, so the fallback path must stay tested.
• Extend KsaBuildVersionProviderTests for both sources and their precedence.

Not blocking anything; the current provider works. Filed so it does not get lost.