Sync updates from internal repository

[atg-service-acct]

Sync updates from internal repository

This PR synchronizes changes from the internal development repository to the public GitHub repository.

Changes

• New samples: VisibilityBufferIndexReorder (augments VisibilityBuffer with index-buffer reordering for Xbox vertex-shader perf)
• Updated samples: Broad refresh across System, Graphics, Live, Audio, Unity, and Tools; ModernGamertag → ModernGamerTag and Haptics main.cpp → Main.cpp case renames; removal of per-sample Desktop.PostBuild.bat scripts
• Updated kits: New AMDTK (AMD FidelityFX); ATGTK (new helpers + relocated imgui/ATG code); OpenSource/imgui, UITK, DirectXTex, DirectXTK12
• General: Large Media/ asset additions (FBX meshes, HDR maps) driving diff size; .gitignore replaced with standard Visual Studio template; removed .configurations/configuration.dsc.yaml ; gdk-samples-list.md updated
• Automated dev-env provisioning was removed (was pinned to the outdated GDK 2303).

[github-actions]

✅ PR Summary Format Eval: PASS

Score: 100/100

PR summary follows the expected format.

Criteria Breakdown

Criterion	Status	Points
H2 Heading	✅	20/20
Bold Summary	✅	25/25
Changes Section	✅	15/15
Bullet List	✅	15/15
Bold Category Prefix	✅	15/15
Recognized Category	✅	10/10

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Format reference: .github/copilot-instructions.md