Let the cold-start comparison excuse a field within a bound

Field entries in the ledger allowlist now apply to every difference row,
in the ledger and in the probe alike, and may carry maxDelta. The traffic
fill gets one with a bound of three: its simulated vehicles enter and
leave every frame, so two pages snapshotted moments apart differ by a
vehicle or two.

scripts/qa-ledger-allow.json

@@ -1,14 +1,18 @@
 [
   {
     "id": "local-dams-footprints",
-    "family": "ground",
     "field": "count",
     "reason": "Infrastructure level of detail keeps features chosen on the way to a view (incumbents and eviction grace, so footprints do not flicker while panning); a cold start reaches the view without that path."
   },
   {
     "id": "local-datacenters-footprints",
-    "family": "ground",
     "field": "count",
     "reason": "Infrastructure level of detail keeps features chosen on the way to a view (incumbents and eviction grace, so footprints do not flicker while panning); a cold start reaches the view without that path."
+  },
+  {
+    "id": "traffic-fill",
+    "field": "count",
+    "maxDelta": 3,
+    "reason": "Simulated vehicles enter and leave the traffic fill every frame, so two pages snapshotted at slightly different moments differ by a vehicle or two."
   }
 ]

scripts/qa/coldStart.mjs

@@ -317,30 +317,16 @@ export function compareColdStart(
     const next = cold.ledger.find(
       (other) => other.family === row.family && other.id === row.id,
     );
-    if (next) {
-      // A field entry excuses one field of one set, with its reason.
-      const rows = compareProbe(
-        { kind: row.kind, count: row.count },
-        { kind: next.kind, count: next.count },
-      ).filter((difference) => {
-        const exception = allow.find(
-          (entry) =>
-            entry.field === difference.field &&
-            entry.id === row.id &&
-            (!entry.family || entry.family === row.family),
-        );
-        if (exception)
-          skipped.push({
-            layer: layerOf(row.family, row.id),
-            family: row.family,
-            id: row.id,
-            ...difference,
-            reason: exception.reason,
-          });
-        return !exception;
-      });
-      add(layerOf(row.family, row.id), row.family, row.id, rows);
-    }
+    if (next)
+      add(
+        layerOf(row.family, row.id),
+        row.family,
+        row.id,
+        compareProbe(
+          { kind: row.kind, count: row.count },
+          { kind: next.kind, count: next.count },
+        ),
+      );
   }
   for (const layer of new Set([
     ...Object.keys(switched.layers),
@@ -369,8 +355,26 @@ export function compareColdStart(
       id,
       compareProbe(a.get(id), b.get(id)),
     );
+  // A field entry excuses one field of one set (in the ledger and the probe),
+  // within maxDelta when it has one, with its reason.
+  const excused = (row) =>
+    allow.find(
+      (entry) =>
+        entry.field === row.field &&
+        entry.id === row.id &&
+        (!entry.family ||
+          entry.family === row.family.replace(/^probe-/, '')) &&
+        (entry.maxDelta === undefined ||
+          Math.abs(Number(row.switchValue) - Number(row.coldStartValue)) <=
+            entry.maxDelta),
+    );
   return {
     differences: differences.filter((row) => {
+      const exception = excused(row);
+      if (exception) {
+        skipped.push({ ...row, reason: exception.reason });
+        return false;
+      }
       const field = sessionState && sessionExclusion(row, switched);
       if (!field) return true;
       skipped.push({ ...row, sessionState: field });