roachtest: add --scatter option to kv workload init for perturbation tests

The kv workload's `init` subcommand supports a --scatter flag that
scatters the table immediately after splitting; this gives the
allocator a chance to spread leaseholders across the cluster while
queues are idle, instead of leaving every split's children with the
parent's leaseholder. Add an opt-in v.scatter field that initWorkload
passes through. Off by default so existing tests are unaffected.

Epic: none
Release note: None

Co-Authored-By: roachdev-claude <roachdev-claude-bot@cockroachlabs.com>

Author: tbg

pkg/cmd/roachtest/tests/perturbation/framework.go

@@ -86,6 +86,7 @@ type variations struct {
 	validationDuration   time.Duration
 	ratioOfMax           float64
 	splits               int
+	scatter              bool // pass --scatter when initializing the workload table
 	timeout              time.Duration
 	numNodes             int
 	numWorkloadNodes     int

pkg/cmd/roachtest/tests/perturbation/kv_workload.go

@@ -24,7 +24,12 @@ func (w kvWorkload) operations() []string {
 }
 
 func (w kvWorkload) initWorkload(ctx context.Context, v variations) error {
-	initCmd := fmt.Sprintf("./cockroach workload init kv --db target --splits %d {pgurl:1}", v.splits)
+	scatterFlag := ""
+	if v.scatter {
+		scatterFlag = " --scatter"
+	}
+	initCmd := fmt.Sprintf("./cockroach workload init kv --db target --splits %d%s {pgurl:1}",
+		v.splits, scatterFlag)
 	return v.RunE(ctx, option.WithNodes(v.Node(1)), initCmd)
 }