docs: fix ETD safe-pace test to a genuinely safe case

docs/superpowers/plans/2026-06-19-etd.md

@@ -41,9 +41,10 @@ mod tests {
 
     #[test]
     fn etd_none_when_on_safe_pace() {
-        // 50% used, 1h elapsed of a 5h window (4h remaining).
-        // Project ~1h more to full → total ~2h « 4h remaining → safe.
-        assert_eq!(etd_secs(50.0, 4 * 3600, 5 * 3600), None);
+        // 10% used, 1h elapsed of a 5h window (4h remaining): steady pace at 1h
+        // is 20%, so 10% is UNDER pace → would not deplete before reset.
+        // (50% in the first hour would be at-risk, not safe — see the invariant.)
+        assert_eq!(etd_secs(10.0, 4 * 3600, 5 * 3600), None);
     }
 
     #[test]