fix(ui): stop Button throwing away a click when something re-renders

[pclauss123]

Summary

Button throws away a click when anything re-renders mid-press. This fixes it.

A click is only dispatched when mousedown and mouseup land on the same
element. The button's inner surface was a component declared insideButton:

constContentWrapper=({ children }: {children: ReactNode})=>(// ← new type every render

A component declared during render is a new component type on every render, so React
unmounts the content subtree and mounts a fresh one each time. Any render occurring
between the two halves of a press replaces the element the pointer is over, and the
click never happens. A hover handler on an ancestor is enough to cause that render,
which is why it presented as intermittent.

Nothing said anything was wrong. The button stayed in the tree, matched by role and
name, and reported enabled throughout. It cost two long debugging sessions in the part
viewer before the cause was found, and it is currently eating the Delete button in
the DFM app — one e2e failure there traces to exactly this.

The surface is now an element built by a plain function rather than a component
declared during render, so its identity survives.

On the test

Asserted as node identity across a re-render, not by firing a click. That is
deliberate: fireEvent.click dispatches the event straight at the element, and jsdom
does not build a click out of mousedown and mouseup — so a click-based test passes
whether or not the surface was replaced. I wrote that version first and confirmed it
passes against the broken component, which is worth knowing if anyone reaches for it
again.

Verified the test fails against the current main component and passes against the
fix:

git checkout origin/main -- src/button/button.tsx && npx vitest run
AssertionError: expected <div …(1)></div> to be <div …(1)></div> // Object.is equality
Tests 1 failed | 4 passed (5)

Public package release

• This PR changes no public package source.
• I ran pnpm changeset and committed the generated Changeset.
• This source change intentionally needs no release; I applied the
  no-release-needed label and explained why below.

patch — a bug fix, no API change.

Validation

• I ran the relevant checks and tests.

pnpm --filter @toolpath/ui test 2 files, 5 tests, green
pnpm --filter @toolpath/ui check-types clean
pnpm --filter @toolpath/ui build clean, dist/ emitted
node scripts/check-release-intent.mjs origin/main
Release intent recorded for: @toolpath/ui

Independent of #51 — different package, no overlap. Either can land first.