Prepare v0.7.3 color range hotfix

[Balragon]

Summary

• Add the v0.7.3 changelog entry for the Vortice GPU color range fix merged in [codex] Fix Vortice video processor color range #43.
• Document the runtime diagnostic override IMIRROR_D3D_COLOR_SPACE.
• Preserve the validation evidence from local build/test and real-device Mac sender visual testing.

Validation

• Code fix already passed CI on [codex] Fix Vortice video processor color range #43.
• Local dotnet build .\iMirror.sln -c Release --no-restore passed with zero warnings and zero errors.
• Local dotnet test .\MacMirrorReceiver.Tests\MacMirrorReceiver.Tests.csproj -c Release --no-restore passed: 63 tests.
• Real-device Mac sender visual validation passed on the Vortice GPU path.

Release

After this merges, tag v0.7.3 on the merge commit to trigger release.yml.