Support portable KSP build paths

[Sean-Kenneth-Doherty]

Summary

• Let the project discover either KSP_x64_Data/Managed or KSP_Data/Managed from the configured KSP root.
• Support KSP_PATH, KSPDIR, KSP_DIR, or direct KSPManagedDir build properties, with clear missing-reference errors.
• Allow compile-only builds with RunPostBuildCopy=false and fix the checked-in AVC version file so it parses as strict JSON.

Why

Current master has already moved the plugin source and version metadata to KSP 1.12.x, but the project still assumes the Windows KSP_x64_Data managed-assembly layout. That blocks a clean build against a normal Linux KSP 1.12.5 install, where the assemblies live under KSP_Data/Managed.

The tracked PAPIPluginContinued.version also has a trailing comma in the GITHUB block, so strict JSON tooling rejects it.

Scope / caveats

This is build-system and metadata cleanup only. It does not change plugin runtime behavior. The default post-build copy still uses the existing Windows xcopy command; Linux or compile-only builds should pass /p:RunPostBuildCopy=false. KSP was not launched and runway-light behavior was not tested.

Validation

• xbuild PAPIPlugin.sln /p:Configuration=Release /p:KSP_PATH="/path/to/Kerbal Space Program" /p:RunPostBuildCopy=false /verbosity:minimal against a local KSP 1.12.5.3190 install.
• xmllint --noout PAPIPlugin/PAPIPlugin.csproj
• jq empty assets/GameData/PAPIPlugin/PAPIPluginContinued.version
• git diff --check HEAD~1..HEAD