refactor: Replace SortableJS 3/4: drop SortableJS from SlickDraggableGrouping (native drag & drop)

[6pac-ai]

Context

Part 3 of the SortableJS removal (port of slickgrid-universal's refactor(core)!: drop SortableJS dependency #2634). Stacked on #1242 (which is stacked on #1240). After this PR nothing in src/ uses SortableJS anymore; the dependency itself and the example <script> tags are removed in the final part.

The header→dropzone characterization spec from #1240 (example-draggable-grouping-header-drop.cy.ts) plus both existing draggable-grouping specs pass unchanged against this port — that was the acceptance gate.

What's in here

• src/slick.interactions.ts — new setupDropzonePillDrag() (ported from Slickgrid-Universal): native HTML5 reordering of group pills inside the dropzone, acceptance of column-header drops via the dataTransfer column id, and the same Firefox/Linux mouse + touch fallbacks as the column reorder engine. Registered on the Slick namespace for iife/browser builds.
• src/plugins/slick.draggablegrouping.ts — fully off SortableJS:
  • getSetupColumnReorder() wires setupColumnReorderDrag (grid container/viewport resolved through public grid getters), with the dropzone show/hide UI choreography moved into the engine's onDragStart/onDragEnd/onDrop callbacks;
  • the dropzone runs on setupDropzonePillDrag; the hand-rolled addDragOverDropzoneListeners() is gone (engine callbacks cover it);
  • grouping pills are created with draggable = true (required for the native HTML5 pill drag);
  • a header drop is handled by both the dropzone's native drop handler and the reorder engine's fallback onDrop path, deduplicated by handleGroupByDrop's existing columnAllowed guard (the fallback path is the only one that fires on Firefox/Linux and touch).

Breaking change

getSetupColumnReorder() now returns { columnReorderDragInstance } (a { destroy() } instance) instead of { sortableLeftInstance, sortableRightInstance }. The grid itself never consumed this return value; only external callers holding the SortableJS instances are affected.

Behavior deltas (all deliberate, noted in-code)

• No more spurious onColumnsReordered after a grouping drop (the old SortableJS onEnd fired it with an unchanged order after every cross-list drop).
• Grouping pages now respect unorderableColumnCssClass for header reordering (the old plugin's filter was commented out, so non-reorderable columns could be dragged there).
• Pill drags only reapply the grouping when the pill order actually changed.

Verification

• Full 47-spec Cypress suite: 600/601 passing, 0 failures (1 intentional pending).
• Draggable-grouping specs (20 + 21 tests), the header→dropzone characterization (3), and both column-reorder characterization specs (8 + 7) all pass unchanged.
• dist/browser/plugins/slick.draggablegrouping.js greps clean of SortableJS after a rebuild.
• Lint and strict TS declaration build clean.

Next (final) part: remove the sortablejs dependency and sweep the example pages.

🤖 Generated with Claude Code