A test proves the served page has exactly one STOP, no layout rule hides it, and it stops on pointerdown · arm

Part of #91, without a browser: the repository has no browser dependency, and adding one is not in scope.

Done:

[ ] New Software/so101/src/stop-layout.test.ts fetches / from a test server and asserts exactly one [data-stop] element.
[ ] It asserts that no CSS rule in the page, in any @media block (portrait, landscape, desktop), sets display:none or visibility:hidden on [data-stop] or on an element that contains it.
[ ] It asserts that the STOP handler is bound to pointerdown, not click.
[ ] Test-only: no change outside the new file. bun --cwd Software/so101 test passes.

A test proves the served page has exactly one STOP, no layout rule hides it, and it stops on pointerdown #129