feat: support multiple independent Board instances (v0) [WIP]

[ernst-dev]

Description

[WIP / first-cut v0] Enables multiple independent Board instances on a single page.

Today the drag-and-drop DragAndDropController is a module-level singleton shared by every Board/ItemsPalette on the page (src/internal/dnd-controller/controller.ts). All droppables register into one shared registry, so collisions and drops "leak" between boards and independent boards cannot be isolated.

This change makes the DnD controller context-scoped and adds an opt-in provider:

• DragAndDropController is now exported and the DnD hooks (useDragSubscription, useDraggable, useDroppable) read their controller from a new React context (DndControllerContext).
• The context defaults to the existing global singleton, so all current usages are unchanged (fully backward-compatible).
• New internal DndContextProvider mints a dedicated controller per subtree.
• New public, opt-in BoardDndProvider component wraps board components in an isolated DnD context:
  • Wrap each board in its own BoardDndProvider → boards are fully isolated.
  • Wrap several boards (and/or a palette) in one BoardDndProvider → they share a context and items can be dragged between them.
  • No provider → unchanged page-wide behavior.
• The provider wrapper uses display: contents so it is layout-neutral.

This is additive and opt-in. It is a coherent, buildable v0 to unblock the epic; it is not feature-complete (see remaining work).

Related links, issue #, if available: Tracking SIM AWSUI-56573.

⚠️ Ticket mismatch: AWSUI-56573 in Taskei currently resolves to an unrelated "Aperture Form Submission" (a Console visual-appearance opt-out feedback entry), not the Board multi-instance epic. Implemented per the assigned epic description; please re-point the tracking ID if needed.

Done in this PR (v0)

• Context-scoped DnD controller with singleton fallback (backward-compatible).
• DndContextProvider (internal) + public BoardDndProvider (+ interfaces, styles, index export).
• Test-utils wrapper (findBoardDndProvider / findAllBoardDndProviders).
• Dev page: pages/board-dnd-provider/multi-instance.page.tsx (isolated boards + shared group).
• Unit tests for context isolation and per-instance controller registry.
• Regenerated documenter + test-utils API-doc snapshots for the new component.

Remaining work (follow-ups, out of scope for v0)

• Functional/visual e2e coverage proving pointer + keyboard DnD isolation across instances (drag between vs. isolate).
• Keyboard/screen-reader insertion across boards within a shared provider; announcements review.
• Auto-scroll / global drag-state styles are still partly global — audit for per-context scoping.
• Public API documentation (Usage tab) and examples.
• Decide the final public surface (dedicated BoardDndProvider vs. a prop on Board) and naming with the team.
• SSR/multiple-React-context edge cases; per-instance item identity guarantees when IDs collide across boards.

How has this been tested?

• npm run build — green.
• npm run test:unit — 336 passed / 34 files (incl. new dnd-context tests; snapshots regenerated).
• eslint . — 0 errors (pre-existing warnings only).
• Manual: new dev page renders two isolated boards + one shared palette/board group.
• Not run here: test:functional / test:visual (Puppeteer e2e) — tracked as follow-up.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates. (partial — API docs follow-up)
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md. (yes — context defaults to the existing singleton)
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines. (follow-up)

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function. (n/a)

Testing

• Changes are covered with new/existing unit tests? (yes)
• Changes are covered with new/existing integration tests? (follow-up)

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.17%. Comparing base (f388073) to head (5672125).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #448      +/-   ##
==========================================
+ Coverage   91.44%   92.17%   +0.73%     
==========================================
  Files          59       62       +3     
  Lines        1730     1751      +21     
  Branches      467      468       +1     
==========================================
+ Hits         1582     1614      +32     
+ Misses        148      137      -11     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.