Measure tool should wait for section plane selection.

[BradEstey]

Summary

• <MeasureTool> mounted beside a <SectionTool> with no cut yet offers no snap and places no point until a cut is chosen or the section tool is unmounted. Escape clearing the cut puts it back to waiting.
• The click that chooses the cut places no measurement point: a release only counts when the measure tool also saw its press.
• SectionStore gains isPicking and setPicking, counted like isEngaged, raised by the section tool's picker.

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, check-types, pnpm lint, pnpm knip, and the example's Playwright suite pass. The new browser test fails against the snapper-hiding change alone and passes with the press guard.