fix(js): make performance.now() monotonic

The old sub-millisecond bump (_sub += 0.1) was uncapped, so after >10 calls in
one millisecond it exceeded 1.0; when the wall clock then ticked over and the
bump reset to 0, now() returned a value below the previous call. Track the last
returned value and never go below it (advance by a tiny epsilon when the clock
has not moved past it).

Fixes #497

crates/obscura-js/src/runtime.rs

@@ -1317,6 +1317,19 @@ mod tests {
         rt
     }
 
+    #[test]
+    fn performance_now_is_monotonic_under_bursty_calls() {
+        let mut rt = setup_runtime("<html><body></body></html>");
+        // Hammer performance.now() so many calls land in the same millisecond and
+        // the wall clock rolls over repeatedly; the value must never go backwards.
+        let violations = rt
+            .evaluate(
+                "(function(){var prev=-Infinity, bad=0; for(var i=0;i<500000;i++){var t=performance.now(); if(t<prev) bad++; prev=t;} return bad;})()",
+            )
+            .unwrap();
+        assert_eq!(violations.as_f64(), Some(0.0), "performance.now() went backwards");
+    }
+
     #[test]
     fn test_document_title() {
         let mut rt = setup_runtime("<html><head><title>Test</title></head><body></body></html>");