feat(dia.Paper): add focus and blur events; expose them as @joint/react Paper props

[kumilingus]

Description

Adds focus/blur paper events with the cell: / element: / link: variants, following the mouseover/mouseout pattern (same guard, same findView resolution, cell:* fired before element:*/link:* via the view notify chain), and exposes them in @joint/react as Paper event props.

The paper events are driven by the bubbling focusin/focusout DOM events — plain focus/blur do not bubble and so cannot be delegated (the same reason React's own onFocus/onBlur use focusin/focusout under the hood). The event names describe what happened to the cell. normalizeEvent is not applied (nothing to normalize on focus events). No blank: variants — a focus event outside of a cell view carries no useful information.

Motivation

Making cells keyboard-focusable (tabindex on the root via attrs) is a common accessibility approach; reacting to the focus currently requires a native DOM listener on paper.el. With this change:

paper.on('element:focus', (elementView) => {
    // e.g. select the focused element
});

or in React:

<Paper onElementFocus={({ id }) => setSelection(id)} />

Changes

@joint/core

• dia/Paper.mjs — focusin/focusout in the events hash + handlers
• dia/CellView.mjs, dia/ElementView.mjs, dia/LinkView.mjs — focusin/focusout methods notifying the cell:/element:/link: focus/blur events
• types/dia.d.ts — the 6 new event signatures
• test/jointjs/paper.js — event order + no event outside of cell views (native bubbling FocusEvents; the test $.fn.trigger shim dispatches non-bubbling CustomEvents)
• changeset (@joint/core: minor)

@joint/react

• presets/paper.ts — the ReactPaper focusin .joint-cell delegation workaround is deleted; the core now emits cell:focus/cell:blur natively under the same names, so the existing onCellFocus/onCellBlur props keep working unchanged (no API break)
• presets/paper-events.ts — new onElementFocus, onElementBlur, onLinkFocus and onLinkBlur props for the element:/link: variants
• paper-focus-events.test.tsx — coverage for the element/link variants
• changeset (@joint/react: minor)