feat(context-menu): verify long-press contextmenu on touch and codify priority order

[OysteinAmundsen]

Sub-issue of #302 — Category E.

Goal

Verify that ContextMenuPlugin already works on touch via native long-press → contextmenu synthesis, fix any preventDefault that suppresses it, and codify the long-press priority order across plugins.

Background

Modern browsers fire contextmenu from:

• Right-click (mouse)
• Two-finger tap (macOS / Windows / iPadOS trackpad)
• Long-press (iOS Safari, Android Chrome, Windows touchscreen) — provided the page doesn't preventDefault() touchstart or pointerdown on the target

libs/grid/src/lib/plugins/context-menu/ContextMenuPlugin.ts:523 already listens for contextmenu, so trackpad two-finger tap and right-click work today. The unknown is whether finger long-press fires contextmenu on grid cells, or whether something in cell event delegation is suppressing it.

This sub-issue is mostly verification + documentation. Likely zero or near-zero net code.

In scope

Verification

• Add Playwright touch-emulation tests that perform a 600 ms long-press on:
  • A grid cell → expect contextmenu event + ContextMenuPlugin menu visible
  • A grid row → expect same (when ContextMenuPlugin has row-scoped items)
  • A header cell → expect the header menu (feat(plugin): column-header-menu plugin (sort, filter, hide, group, pin, autosize) #270) opens instead; contextmenu on header is overridden
• If long-press does NOT fire contextmenu on cells today, locate the offending preventDefault() (likely in event-delegation.ts or RowDragDropPlugin) and narrow it.

Long-press priority order — codify and document

Per #302's resolution policy:

Target	Long-press action	Condition
Header	Open header menu (#270)	Always (when #270 registered)
Row	Enter row selection mode	When SelectionPlugin mode includes 'row'
Row	Open ContextMenuPlugin	Otherwise (or via "More…" inside selection-mode toolbar)
Cell	Enter cell range-paint mode	When SelectionPlugin mode includes 'cell' or 'range'
Cell	Open ContextMenuPlugin	Otherwise (or via "More…" inside selection-mode toolbar)

Document this priority order in:

• ContextMenuPlugin.ts JSDoc (class-level)
• apps/docs/src/content/docs/grid/guides/touch-input.mdx (cross-cutting sub-issue owns this file)
• ContextMenuPlugin.mdx plugin docs page

ContextMenuPlugin "More…" item in selection-mode toolbar

When both ContextMenuPlugin and SelectionPlugin are registered AND selection-mode is active (Category C), SelectionPlugin's toolbar surfaces a "More…" button that opens ContextMenuPlugin's menu — this preserves right-click parity even when long-press is consumed by selection mode.

This wiring is implemented in Category C's sub-issue; this sub-issue only documents the contract.

Out of scope

• Implementing a long-press polyfill — modern browsers handle it natively.
• Custom long-press timing per device class (single 400 ms threshold for selection mode entry; native browser handles contextmenu timing, typically ~500 ms).
• Selection-mode toolbar implementation — Category C.

Acceptance criteria

• Playwright touch-emulation test confirms long-press fires contextmenu on cells when ContextMenuPlugin is registered alone (no SelectionPlugin).
• Same test confirms long-press fires contextmenu on rows when applicable.
• If a preventDefault() is currently suppressing native long-press → fix narrowed; before/after test demonstrates the regression-fix.
• ContextMenuPlugin.ts class-level JSDoc documents the long-press priority order.
• apps/docs/src/content/docs/grid/plugins/context-menu.mdx (or equivalent) updated with a "Touch input" section linking to the touch guide.
• No regression on existing right-click and Shift+F10 keyboard handling (context-menu.spec.ts passes).

Dependencies

• Touch guide MDX file exists — cross-cutting sub-issue.
• For full integration testing: Category C sub-issue (selection-mode toolbar with "More…" item) and feat(plugin): column-header-menu plugin (sort, filter, hide, group, pin, autosize) #270 (header menu).

References

• Parent epic: [Epic] feat(grid): touch / pointer-input parity audit & implementation #302
• ContextMenuPlugin.ts:523 — current contextmenu listener
• ContextMenuPlugin.ts:584 — existing Shift+F10 keyboard handler