Merge ticket 040 (unify rate/window implementation)

Fix one test call site to unwrap the Result now returned by
evaluate_promql (introduced by ticket 041). Full suite (194 tests),
clippy, and webapp build all pass after merging all of 037, 040, 041,
042, and 043.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/engine.rs

@@ -2348,7 +2348,7 @@ mod tests {
 
         // Dashboard side: rate() over a 2s window, read at the series' own last timestamp.
         let parsed = promql::parse_query("rate(requests[2s])").unwrap();
-        let series = evaluate_promql(&index, &parsed, None, None);
+        let series = evaluate_promql(&index, &parsed, None, None).unwrap();
         assert_eq!(series.len(), 1);
         let (_, dashboard_rate) = *series[0].points.last().unwrap();