Show one seed column when the flow declares a seed input

The run-level seed is that parameter then, so the comparison table drew it
twice and counted it twice in "N of M varied".

frontend/src/components/Runs/RunsScreen.tsx

@@ -724,12 +724,15 @@ function ParamDiff({ rows }: { rows: RunRow[] }) {
   const one = rows.every((run) => run.flow === rows[0].flow)
   const { declared } = useFlowInputs(one ? rows[0].flow : undefined)
   const varying = varyingKeys(rows)
-  // Not a parameter, but it is part of what produced the number, and in a
-  // sweep it is often the only thing that moved.
-  const seedVaries = new Set(rows.map((run) => run.seed ?? null)).size > 1
   const keys = new Set<string>()
   for (const run of rows)
     for (const key of Object.keys(run.params)) keys.add(key)
+  // Not a parameter, but it is part of what produced the number, and in a
+  // sweep it is often the only thing that moved. Unless the flow declares a
+  // `seed` input: the run's seed is then that parameter, already a column.
+  const seedColumn = !keys.has("seed")
+  const seedVaries =
+    seedColumn && new Set(rows.map((run) => run.seed ?? null)).size > 1
   const shared = keys.size - varying.length
 
   if (varying.length === 0 && !seedVaries) {
@@ -747,7 +750,8 @@ function ParamDiff({ rows }: { rows: RunRow[] }) {
       <div className="flex flex-wrap items-baseline gap-x-3">
         <h2 className="font-medium text-sm">What differs</h2>
         <p className="text-muted-foreground text-xs">
-          {varying.length + (seedVaries ? 1 : 0)} of {keys.size + 1} varied
+          {varying.length + (seedVaries ? 1 : 0)} of{" "}
+          {keys.size + (seedColumn ? 1 : 0)} varied
           {shared > 0
             ? shared === 1
               ? "; the other one is the same across all of them"