spike(v4): add GPU decoder decision probes

[YashasVM]

What changed

• adds isolated C++20 Media Foundation and FFmpeg D3D11 texture probes under spikes/gpu
• exercises explicit-LUID decode plus D3D11 scale, colour conversion, rotation, and four-input 2x2 composition
• adds bounded workload definitions, result validation, orchestration, ETW capture guidance, behavioural tests, and raw hardware smoke evidence
• records an ADR that deliberately leaves decoder selection blocked

Why

V4-02 needs native, fail-closed probes before either decoder can be selected. Runtime-only FFmpeg capability output is not sufficient evidence.

Evidence and limitations

• 43 Python tests pass
• both native targets compile with MSVC and both CTest smoke tests pass
• five-second hardware smoke matrix passes for H.264/HEVC at 4x1080p30 and 2x1080p60 on RTX 4050 LUID 0x0:0x16933
• one-hour resource/latency runs, ETW copy corroboration, genuine device-removal with ingress/ISO continuity, and hardware programme encode are explicitly NOT_RUN
• no decoder is selected and no product compositor or engine code is added

Rollback

Remove spikes/gpu, tests/test_v4_gpu_spike.py, the V4-02 ADR, and its smoke-evidence file. No product path depends on this spike.

[coderabbitai]

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