feat(joint-core, joint-react): PointerEvents drag survival + capture in React preset

[kumilingus]

Summary

• joint-core: new public Paper#setDragging(evt, value=true) / Paper#isDragging(evt) API. Joint-core itself sets the flag from every action-confirmed drag-start branch (element / link / label / arrowhead / magnet→link). External consumers can ask "is a drag actually happening?" without poking view eventData internals.
• joint-react: paper drag-survival migrates to real PointerEvent (pointermove / pointerup / pointercancel) and acquires setPointerCapture on the original pointerdown target on the first pointermove of a confirmed drag. Drags now survive the pointer leaving the host.

Joint-core's events hash is unchanged — mousedown / touchstart still drive pointerdown. Only documentEvents (the drag-survival layer) is swapped to pointer events in the React preset.

joint-core changes

• dia/Paper.mjs — add setDragging / isDragging near delegateDragEvents. No callers outside the new API methods.
• dia/ElementView.mjs — dragStart calls paper.setDragging(evt) after can('elementMove') passes.
• dia/LinkView.mjs — dragStart, dragLabelStart, dragArrowheadStart all call paper.setDragging(evt) inside their respective can(\u2026) gates.
• dia/CellView.mjs — dragLinkStart calls paper.setDragging(evt) once the magnet\u2192link transition actually creates the link (covers the magnet path that defers via magnetThreshold='onleave').
• types/dia.d.ts — declare both methods.

joint-react changes

packages/joint-react/src/presets/paper.ts:

• documentEvents overridden to { pointermove, pointerup, pointercancel }.
• startListening extended to record pointerTarget on cell:pointerdown + element:magnet:pointerdown (magnet event fires first on passive magnets; guard prevents the later cell event from overwriting).
• pointermove override: when paper.isDragging(event) passes and captureTarget not yet set, calls setPointerCapture(pointerId) on data.pointerTarget (falls back to paper.el) and stashes captureTarget. Adds jj-is-dragging class to paper.el.
• pointerup override: releases capture on captureTarget, clears jj-is-dragging class.

packages/joint-react/src/css/styles.css:

• New CSS vars --jj-drag-cursor, --jj-dragging-cursor, --jj-port-drag-cursor, --jj-port-dragging-cursor.
• .jj-is-dragging .jj-box, .jj-is-dragging .jj-port-body[magnet=\"active\"] cursor rules gated on the dragging class.

Test plan

• yarn workspace @joint/react jest --testPathPatterns=\"paper\" — 85/85 pass.
• yarn workspace @joint/react typecheck clean (modulo the pre-existing code-controlled-mode-reset-bug.tsx height regression already on dev).
• Live smoke (storybook):
  • Drag an element — captured, drag follows the pointer when it leaves paper bounds, drops at release point.
  • Drag a link / link label — same behavior.
  • Drag a magnet → creates a link — capture acquired after the magnet threshold; arrowhead follows the pointer beyond paper bounds.
  • Right-click on cells/blank — no drag, contextmenu fires normally.

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