Add GPU_FOREIGN_UTIL_PCT: game detection via per-process GPU 3D-engine usage

Reads the same PDH counters as Task Manager (\GPU Engine(*)\Utilization
Percentage, locale-independent via PdhAddEnglishCounterW), which cover
graphics work under WDDM — games are caught without an exe list and the
offender is named. Windows-only; a missing or failing counter disables
the path with one log line. dwm (the desktop compositor) joins the
default ignore list.

internal/game/pdh.go

@@ -0,0 +1,36 @@
+package game
+
+import (
+	"errors"
+	"strconv"
+	"strings"
+)
+
+// errNotPrimed marks the first PDH sample after opening a query: rate-based
+// counters (like engine utilization) need two collections before they
+// return meaningful values.
+var errNotPrimed = errors.New("GPU engine counter needs a second sample")
+
+// parseGPUEngineInstance splits a PDH "GPU Engine" instance name —
+// "pid_1234_luid_0x00000000_0x00011A2B_phys_0_eng_0_engtype_3D" — into PID
+// and engine type ("3D", "Copy", "VideoDecode", ...). engType is empty when
+// the name carries no engtype marker.
+func parseGPUEngineInstance(name string) (pid int, engType string, ok bool) {
+	rest, found := strings.CutPrefix(name, "pid_")
+	if !found {
+		return 0, "", false
+	}
+	digits, rest, found := strings.Cut(rest, "_")
+	if !found {
+		return 0, "", false
+	}
+	pid, err := strconv.Atoi(digits)
+	if err != nil || pid < 0 {
+		return 0, "", false
+	}
+	const marker = "engtype_"
+	if i := strings.LastIndex(rest, marker); i >= 0 {
+		engType = rest[i+len(marker):]
+	}
+	return pid, engType, true
+}