Resample a sectioned part while the measure tool is active.

[BradEstey]

Summary

• While <MeasureTool> is mounted, a part cut by a section is measured as it is seen. The tool
  re-samples the part whenever a clipping plane moves: the half the cut removed is not offered
  for snapping, the part's edges stop at the plane, the outline where the plane passes through
  solid material is an edge with corners of its own, and the capped face is a surface a point
  can land on.
• The plane is read off the part material's clipping planes, so it works for the viewer's own
  cut and for a controlled section prop alike. The sample is keyed on the plane and cached per
  mesh, so a handle drag or a depth change re-samples on the next pointer move and a still cut
  costs nothing.
• sectionContour, clipSegments, pointInContour, capHit and sampleSection are the pure
  functions behind it, exported from the root.
• The example gains a toy brick model, hollow with tubes underneath, for cutting open and
  measuring.

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.

Validation

• I ran the relevant checks and tests.

• pnpm --filter @toolpath/viewer test — 305 passed

• pnpm --filter @toolpath/example-react-viewer test — 15 passed (Playwright), including a new
  test that measures from the cap and again after the cut is deepened

• pnpm check-types, pnpm exec eslint, pnpm knip — clean