perf: replace setTimeout drag with optimised RAF split engine

[SumitRajpal]

Summary

Replaces the original setTimeout(100ms) drag implementation with a fully
optimised RAF-based split engine, delivering significantly smoother resizing
across all devices.

What changed

New files

• src/components/splitEngine/useDragEngine.js — standalone drag engine hook
• src/components/splitEngine/clamp.js — pure arithmetic utility

Modified

• src/components/Resizer.jsx — wired to new engine, DEV-only FPS overlay
• src/components/Layout.jsx — passes layoutRef as containerRef to Resizer

Optimisations applied

#	Optimisation	Impact
1	CSS variable bypass	onMove → React reconcile drops from 60/sec to 1 (on drop only). DOM writes go direct to GPU thread.
2	Stable V8 hidden class	drag object never set to null — V8 holds one IC forever, no deopt.
3	setPointerCapture	Browser skips hit-testing on every pointermove at 60fps.
4	getCoalescedEvents()	No sub-frame pointer input silently discarded between RAF frames.
5	Closure-free hot path	All values live in engineRef. handleMove has zero deps and is never re-JIT'd by V8.

DEV-only FPS overlay

A fixed bottom-right FPS counter is shown during active drag in development
mode only (import.meta.env.DEV). Stripped completely from production builds.

Test plan

• Drag the resizer left and right — confirm smooth movement
• Drag past left/right thresholds — confirm display switches to chart/grid
• Release drag — confirm grid snaps to correct final position
• Collapse/expand buttons still work in both compact and normal mode
• FPS overlay appears bottom-right in dev, absent in prod build
• Touch/stylus drag works (pointer events)