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.
Claude