perf(cuesheet): add temporary first-render metrics scaffold

Adds a dev-only, self-contained instrumentation module under
common/devtools/cuesheet-metrics to measure the first-render (mount) cost
of virtualised cuesheet rows, so scroll-performance changes can be backed
by before/after numbers.

- Console-only API on window.__cuesheetPerf (deterministic scroll benchmark
  + manual start/stop), gated by isDev && ?perf=1, runtime-inert in prod.
- Whole-row mount probe plus per-subsystem attribution: colour calc,
  getVisibleCells, AutoTextarea autosize, Tooltip portal mount,
  useReactiveTextInput hotkey-handler init; FPS/long-frame timing.
- All edits to existing files tagged // PERF-METRICS for a one-pass teardown.

Temporary: to be deleted once the optimizations are proven.

apps/client/src/common/devtools/cuesheet-metrics/usePerfMark.ts

@@ -0,0 +1,38 @@
+/**
+ * PERF-METRICS — temporary cuesheet scroll-performance scaffold (delete with this directory).
+ *
+ * Lightweight timing helpers. All no-op when instrumentation is disabled so the call sites in
+ * shared components stay cheap (and fold away in production builds).
+ */
+import { useLayoutEffect, useRef } from 'react';
+
+import { PERF_ENABLED } from './perfConfig';
+import { recordMark, recordRowMount } from './perfStore';
+
+/** Times the synchronous `fn`, records the duration under `name`, and returns its result. */
+export function timeSync<T>(name: string, fn: () => T): T {
+  if (!PERF_ENABLED) return fn();
+  const start = performance.now();
+  try {
+    return fn();
+  } finally {
+    recordMark(name, performance.now() - start);
+  }
+}
+
+/**
+ * Measures the mount cost of a component (render-start → post-commit layout) and records it
+ * under `${name}.mount`. Used on EventRow and on the Tooltip subtree to attribute per-row
+ * first-render cost. Pass `countAsRow` on the row probe so only rows feed the rows-mounted tally.
+ */
+export function useMountProbe(name: string, countAsRow = false): void {
+  const renderStart = useRef(PERF_ENABLED ? performance.now() : 0);
+  useLayoutEffect(() => {
+    if (!PERF_ENABLED) return;
+    recordMark(`${name}.mount`, performance.now() - renderStart.current);
+    if (countAsRow) {
+      recordRowMount();
+    }
+    // eslint-disable-next-line react-hooks/exhaustive-deps -- mount-only probe
+  }, []);
+}