M1: X11 active-window adapter + platform fallback

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/evidence/source_other.go

@@ -0,0 +1,16 @@
+//go:build !linux
+
+package evidence
+
+import "context"
+
+// NewSource returns a sensor that reports evidence permanently unavailable on
+// platforms without the X11 adapter.
+func NewSource() Source { return noopSource{} }
+
+type noopSource struct{}
+
+func (noopSource) Watch(ctx context.Context, onChange func(WindowSnapshot)) {
+	onChange(WindowSnapshot{Health: EvidenceHealth{Available: false, Reason: "no active-window sensor on this platform"}})
+	<-ctx.Done()
+}