Set up Linux dev environment and restore buildability

[wren11]

What this does

• installs and validates a Linux-compatible toolchain path for this legacy .NET 3.5 WinForms solution (Mono-based)
• makes the repository buildable in this environment by adding missing in-repo pathfinding source files and a fallback BlackMagic implementation when the proprietary DLL is unavailable
• updates project file conditions so the fallback is used only when the external DLL is absent

Why

The checkout could not be built/run as-is because:

• WrenBot/PathFinding/*.cs files referenced by WrenBot.csproj were missing from the repository
• BlackMagic.dll was referenced from a local desktop path and was not available in cloud/Linux environments

Validation

• xbuild WrenBot.sln /p:Configuration=Debug now succeeds on this VM.
• Runtime demonstration artifacts:
  • wrenbot_environment_demo_success.mp4
  • WrenBot foreground window
  • WrenBot responsive click
  • wrenbot_runtime.log

_{To show artifacts inline, enable in settings.}