Add emit-on-change tracker for the Windows polling sensor

internal/evidence/foreground_tracker.go

@@ -0,0 +1,21 @@
+package evidence
+
+// foregroundTracker remembers the last observed foreground window so the
+// Windows polling Source emits only on change. hwnd is held as uintptr so this
+// file stays platform-neutral (it must build and test on Linux).
+type foregroundTracker struct {
+	primed    bool
+	available bool
+	hwnd      uintptr
+	title     string
+}
+
+// changed reports whether (available, hwnd, title) differs from the last
+// observation and records the new values. The first call always returns true.
+func (t *foregroundTracker) changed(available bool, hwnd uintptr, title string) bool {
+	if t.primed && available == t.available && hwnd == t.hwnd && title == t.title {
+		return false
+	}
+	t.primed, t.available, t.hwnd, t.title = true, available, hwnd, title
+	return true
+}

internal/evidence/foreground_tracker_test.go

@@ -0,0 +1,29 @@
+package evidence
+
+import "testing"
+
+func TestForegroundTrackerChanged(t *testing.T) {
+	var tr foregroundTracker
+
+	if !tr.changed(true, 100, "A") {
+		t.Fatal("first observation should always report changed")
+	}
+	if tr.changed(true, 100, "A") {
+		t.Error("identical observation should not report changed")
+	}
+	if !tr.changed(true, 100, "B") {
+		t.Error("title change on same hwnd should report changed")
+	}
+	if !tr.changed(true, 200, "B") {
+		t.Error("hwnd change should report changed")
+	}
+	if !tr.changed(false, 0, "") {
+		t.Error("transition to unavailable should report changed")
+	}
+	if tr.changed(false, 0, "") {
+		t.Error("repeated unavailable should not report changed")
+	}
+	if !tr.changed(true, 200, "B") {
+		t.Error("transition back to available should report changed")
+	}
+}