fix(board): stop phantom text node when dbl-clicking custom node title

[winlp4ever]

Summary

Dbl-clicking a custom node's title (sheet, code-sandbox, widget, mini-app, folder, document, icon, image) spawned a stray text note on the canvas. The title is positioned absolute below the node — outside canvas-harness's hit-test rect — so the native dblclick saw it as empty space and fell through harness-canvas's "create text node" branch.

What's in the diff

• New hook `useStopCanvasDblClick` in use-stop-canvas-gesture.ts — sibling to the existing pointerdown stop. Stops native dblclick on the given element so the canvas-harness wrap never sees it.
• NodeTitleCaption — added a stable `wrapperRef` on the outer `
  ` and bound both stops there. Replaces the previous calls bound to `buttonRef` and `inputRef` (those refs swap on `editing`, and `useEffect` deps don't track `ref.current` mutations, so binding to a conditionally-rendered ref only caught one of the two states).
• Removed dead `buttonRef` and updated stale docstring.

Why `useStopCanvasDblClick` is a SEPARATE hook

React 17+ delegates events at the root container. A native `stopPropagation` below the root also blocks React's synthetic `onDoubleClick` on any descendant. The sheet body relies on its React `onDoubleClick` to open the inline editor (sheet/view.tsx:155) — so we can't blanket-stop dblclick everywhere.

The split lets each callsite pick its level of stopping:

• Bodies (code-sandbox, sheet, etc.) — pointerdown only. React onDoubleClick still fires.
• Title caption — pointerdown + dblclick. The descendant button/input have only `stopPropagation` in their React onDoubleClick — nothing useful is lost.

The new hook's docstring spells out when it's safe to use.

Why the wrapper, not the button/input

`editing` flips between `` and ``. The buttonRef is set on first render; the inputRef is null at first render (since the input isn't mounted yet), and `useEffect` deps `[ref]` don't re-trigger when the input later mounts. So the input never got a listener — and the user's double-click resolves on the input by the time the second click lands. The wrapper `

` is mounted for the component's lifetime, so events from either child bubble through it reliably.

Test plan

• `npm run type-check` clean
• `npm test --run` — 320/320 green
• Dbl-click a custom node's title (try sheet, code-sandbox, widget, mini-app, folder, icon, image) → no phantom text node spawns
• Single-click the title → enters edit mode (unchanged)
• Dbl-click inside the title's input (word-select) → browser default still works
• Dbl-click the sheet body → inline markdown editor still opens (this is the regression the refactor specifically protects against)
• Dbl-click code-sandbox body → opens the panel (unchanged; uses onClick, not onDoubleClick)
• Dbl-click empty canvas → still creates a text node (unchanged)