Read a key a run lacks as its flow's declared value in "What differs"

Runs from before the declared values were folded into params lack them, so
every such key read as varied next to a newer run with the same value.

frontend/src/components/Runs/RunsScreen.tsx

@@ -723,7 +723,7 @@ function ParamDiff({ rows }: { rows: RunRow[] }) {
   // declaration lookup per flow, and a sweep comparison never does.
   const one = rows.every((run) => run.flow === rows[0].flow)
   const { declared } = useFlowInputs(one ? rows[0].flow : undefined)
-  const varying = varyingKeys(rows)
+  const varying = varyingKeys(rows, declared)
   const keys = new Set<string>()
   for (const run of rows)
     for (const key of Object.keys(run.params)) keys.add(key)

frontend/src/components/Runs/queries.ts

@@ -310,14 +310,24 @@ export const shortCommit = (commit: string) => commit.slice(0, 7)
  * What makes a sweep readable: fifty runs of one flow share everything but the
  * two knobs that were swept, and those two are the only columns worth drawing.
  */
-export function varyingKeys(runs: { params: Record<string, unknown> }[]) {
+export function varyingKeys(
+  runs: { params: Record<string, unknown> }[],
+  declared: Map<string, unknown> = new Map(),
+) {
   if (runs.length < 2) return []
   const keys = new Set<string>()
   for (const run of runs)
     for (const key of Object.keys(run.params)) keys.add(key)
+  // A key a run lacks is one it took from its flow, read as what the flow
+  // declares now — the value its cell shows.
+  // ponytail: a run from before 2026-08-29 stored only what was passed, so if
+  // its flow's `initial` changed since, a real difference reads as none. Exact
+  // needs the definition at the run's version, which no route serves.
+  const value = (run: (typeof runs)[number], key: string) =>
+    JSON.stringify(key in run.params ? run.params[key] : declared.get(key))
   return [...keys].filter((key) => {
-    const first = JSON.stringify(runs[0].params[key])
-    return runs.some((run) => JSON.stringify(run.params[key]) !== first)
+    const first = value(runs[0], key)
+    return runs.some((run) => value(run, key) !== first)
   })
 }