perf: add vendor-agnostic GPU monitoring for Windows (experimental) (#779)

Add GPU monitoring support for AMD and Intel GPUs on Windows using
D3DKMT (DirectX) and PDH performance counters.

- Add PDH-based GPU utilization via \GPU Engine(*)\Utilization
Percentage counter, summing all engine types per adapter (3D, Compute,
Copy, Video).
- Add D3DKMT bindings for adapter enumeration, memory segments, and
adapter perf data.
- Use PDH as primary utilization source (works on all vendors), with
D3DKMT RunningTime as fallback for systems without PDH counters.
- Prefer nvidia-smi when available, fall back to D3DKMT + PDH for
AMD/Intel.
- Backend priority: nvidia-smi -> D3DKMT + PDH -> ErrNoGpuTool.

Verified on AMD 7900XTX GPU with llama.cpp Vulkan & ROCm backend: GPU
utilization correctly shows ~99% during inference, ~0-2% when idle.

---

LLM disclosure: GLM 5.1 & Kimi K2.6 have been used extensively during
exploration and coding to the point that the LLM's wrote over 3/4 of the
code, and I have done additional verification myself.
As such, it should be considered experimental.
Additional verification is needed.

I have tested it on my 7900XTX system with Windows 11, and it works
correctly, but as I only have this one rig, I cannot verify it
everywhere.

internal/perf/monitor_windows.go

@@ -22,6 +22,13 @@ func getGpuStats(ctx context.Context, every time.Duration, logger *logmon.Monito
 		logger.Debugf("nvidia-smi: %s", err.Error())
 	}
 
+	if ch, err := tryD3DKMT(ctx, every, logger); err == nil {
+		logger.Info("using D3DKMT for GPU monitoring")
+		return ch, nil
+	} else {
+		logger.Debugf("D3DKMT: %s", err.Error())
+	}
+
 	return nil, ErrNoGpuTool
 }