perf(frontend): preview only when the selected rule actually changed

Raised in review of #61, and my comment was the weaker claim: it said the
trigger fires on each keystroke, when it fires on ANY document mutation.
`rules` is rebuilt whenever `script` is reassigned, so adding, deleting or
reordering some OTHER rule rescheduled a preview of the selected one and
spent a request to be told the same bytes it was already showing.

Comparing the wire payload rather than the array reference makes those
free, and covers the keystroke that leaves wire content unchanged too. The
payload compared is the one that gets posted, so the two cannot drift.

An error clears the remembered payload, so the next mutation retries
rather than matching it and leaving the message up for good.

The comment now says what actually happens.

Not unit-tested: the logic is inside a .svelte file and this repo has no
component test harness. svelte-check, vitest and the build are clean.

Refs areyousievious-8fg.17

Author: derekslenk

frontend/src/routes/RuleEditor.svelte

@@ -39,18 +39,33 @@
   const PREVIEW_DEBOUNCE_MS = 200;
   let previewSeq = 0;
   let previewTimer;
+  // The wire payload the preview on screen was rendered from. The trigger
+  // below fires on ANY document mutation — `rules` is rebuilt whenever
+  // `script` is reassigned, so adding, deleting or reordering some OTHER rule
+  // reschedules the selected one too. Comparing payloads rather than array
+  // references means those cost nothing, and so does a keystroke that leaves
+  // the wire content unchanged.
+  let previewedWire = '';
 
   function schedulePreview(rule) {
-    clearTimeout(previewTimer);
-    const wanted = (previewSeq += 1);
     if (!rule) {
+      clearTimeout(previewTimer);
+      previewSeq += 1;
       preview = '';
       previewError = '';
+      previewedWire = '';
       return;
     }
+    const payload = doc.entryToWire(rule);
+    const identity = JSON.stringify(payload);
+    if (identity === previewedWire) return;
+    previewedWire = identity;
+
+    clearTimeout(previewTimer);
+    const wanted = (previewSeq += 1);
     previewTimer = setTimeout(async () => {
       try {
-        const { sieve } = await api.previewRule(doc.entryToWire(rule));
+        const { sieve } = await api.previewRule(payload);
         if (wanted !== previewSeq) return;
         preview = sieve;
         previewError = '';
@@ -61,6 +76,9 @@
         // in a different shape.
         preview = '';
         previewError = e?.message || 'Preview unavailable';
+        // Forget what we asked for, so the next trigger retries instead of
+        // matching `identity` and leaving the error up forever.
+        previewedWire = '';
       }
     }, PREVIEW_DEBOUNCE_MS);
   }
@@ -92,8 +110,9 @@
 
   $: dirty = !!(script && pristine) && !doc.sameWire(script, pristine);
 
-  // Every document mutation makes a fresh `rules`, so this fires on each
-  // keystroke and the debounce inside does the rest.
+  // Fires on every document mutation, including ones to other rules — see
+  // `schedulePreview`, which is what decides whether the selected rule
+  // actually changed.
   $: schedulePreview(rules[selectedIdx]);
 
   function addRule() {