perf(system): reduce pan-on-scroll gesture work - #5884

Summary

When panOnScroll is enabled, wheel gestures previously repeated avoidable native work on every event:

• the manual pinch path used d3-selection's pointer, which read the pane's bounding rect on every ctrl-wheel tick;
• d3 emitted an end event for every pinch scaleTo, making xyflow clear and recreate its trailing onPanZoomEnd timer on every tick;
• the direct scroll-pan path independently cleared and recreated its own trailing end timer on every tick.

This change caches the pane origin for each 150 ms pinch gesture and coalesces both end paths with deadline-based trailing timers. The latest event and viewport are still delivered once, 150 ms after the final tick. D3 end state remains private per pan/zoom instance in a WeakMap.

Measurement

Call-count probes over the real handlers produced:

A timer wakes and re-arms against the latest deadline during a long gesture. Pointer coordinates and the final viewport were unchanged, and a new gesture after the idle window gets a fresh timer and remeasures the pane origin.

This was motivated by a production profile where the pinch path's repeated pane reads and timer churn accounted for about 308 ms of main-thread self time across a slow zoom gesture. The direct pan timer improvement is a follow-up found during downstream review; it was not part of that 308 ms measurement.

Validation

• pnpm --filter @xyflow/system typecheck
• ESLint and Prettier on the changed files
• pnpm --filter @xyflow/system build
• React pane Playwright suite in Chromium: 8/8 passed
• Browser timer probe: two 100-event pan gestures produced one timer per gesture and zero clears

The package-wide lint command also reports two existing no-unnecessary-type-assertion errors in src/utils/edges/positions.ts on unmodified main.