perf(tooling): reach a fully populated sidebar in both profilers

Idle and streaming cost both depend on how much of the sidebar is mounted, so
the scenario setup is now shared. --expand-projects seeds the persisted collapse
state; --expand-sessions clicks every "Show more sessions" control, which
cannot be seeded because pagination is component state. Both run before the
measured window, so it stays input-free. Session expansion must run after the
sidebar has populated, not straight after the load event, or the controls do
not exist yet.

Also replaces a spread push over collected trace events, which overflowed the
call stack once a populated sidebar produced chunks of hundreds of thousands of
events, and the equivalent spread in the heap-maximum calculation.

scripts/perf/scenario.mjs

@@ -0,0 +1,44 @@
+/**
+ * Scenario setup shared by the idle and streaming profilers.
+ *
+ * Idle and streaming cost both depend on how much of the sidebar is mounted,
+ * so both commands need the same way to reach a heavily populated sidebar.
+ * Setup always runs before the measured window.
+ */
+
+import { evaluateValue, wait } from "./cdp.mjs"
+
+/**
+ * Expands every project. The sidebar persists the ids of collapsed projects,
+ * so an empty list expands everything. Requires a reload to take effect.
+ */
+export const expandProjects = async (client) => {
+  await evaluateValue(client, `localStorage.setItem("oc.sessions.projectCollapse", "[]")`)
+}
+
+/**
+ * Clicks every "Show more sessions" control until none remain.
+ *
+ * Session list pagination is component state, so unlike project collapse it
+ * cannot be seeded through storage. The controls only exist once the sidebar
+ * has populated, so call this after the page has settled, never straight after
+ * the load event.
+ *
+ * Matching is a case-insensitive substring test, which assumes the English UI
+ * locale; a non-English locale expands nothing and reports zero.
+ */
+export const expandSessionLists = async (client, { passes = 40, settleMs = 400 } = {}) => {
+  let totalClicked = 0
+  for (let pass = 0; pass < passes; pass += 1) {
+    const clicked = await evaluateValue(client, `(() => {
+      const controls = [...document.querySelectorAll("button")]
+        .filter((button) => (button.textContent ?? "").toLowerCase().includes("show more"))
+      for (const control of controls) control.click()
+      return controls.length
+    })()`)
+    if (!clicked) break
+    totalClicked += clicked
+    await wait(settleMs)
+  }
+  return totalClicked
+}