DRAFT: Add Zstd GPU CI test infrastructure (Phase 1)

[brohan203]

Draft - not ready for merge. Putting this up for early visibility and code review.

Adds a thin GTest wrapper (zstdgpu_ci_tests) that shells out to zstdgpu_demo.exe to validate Zstd GPU decompression shaders in CI. Also adds pipeline YAML and a histogram generation script for perf results.

What's here

• zstd/zstdgpu_ci_tests/ — GTest parameterized test files
  • main.cpp handles setup (CLI parsing, config, file discovery), and zstdgpu_ci_tests.cpp handles executions (test definitions, demo runner)
  • 4 correctness scenarios: SimulationCheck, D3D12DebugLayer, ExternalMemory, GraphicsQueue
  • 2 performance scenarios: OverallThroughput, PerStageTiming
  • Win32 CreateProcess runner with pipe capture and optional timeout
• zstd/scripts/generate_histogram.py — Generates histogram PNGs from perf CSV output. This can probably be improved

Open Q's

• Do we need x86?
• There's a decent amount of google test infra in the zstd/zstdgpu_samples folder. Need to look into reusing that instead of new methods I wrote

What's not here

• Changes to zstdgpu_demo.exe. ATG owns. This PR does not include --output-csv flag and non-zero exit code changes
• Test data staging on lab machines
• End-to-end pipeline validation

ATG dependencies

• --output-csv flag on zstdgpu_demo.exe for perf CSV output
• Non-zero exit code from zstdgpu_demo.exe when validation fails (correctness tests currently pass vacuously)