Fix demand-loaded building artwork ownership

[Krisztiaan]

Summary

Fix demand-loaded building artwork ownership and deallocation, preventing heap corruption in mods such as TFADW.

Behavior and compatibility

Bug fix for DemandLoad, DemandLoadBuildup, and FreeBuildup. No format, save, replay, networking, determinism, COM, or ABI changes.

Validation

• VS2022 Win32 Debug and Release builds and tests passed.
• Exact Release artifact tested under Wine with TFADW 1.02 and UnitCount=1.
• Skirmish loaded, demand-loaded artwork initialized, and gameplay began without the original crash.
• Native Windows runtime not tested.

Documentation

Updated the affected key documentation, MIX ownership guidance, and change record.

Checklist

• The change is focused; unrelated mechanical cleanup is separate
• Compatibility effects and any migration are explicit
• Validation distinguishes what passed, failed, and was not run
• No prohibited assets, binaries, SDKs, credentials, or generated output are included