Test overflow on pages and fixed artboards #25

Add direct tests for checkOverflow in a new proof/test/overflow.test.mjs, using synthetic DOM boxes and the function in proof/check.mjs.

Cover content below the viewport on a scrolling page versus a fixed artboard, horizontal overflow on both, and a text box clipped by a container even when it stays inside the canvas. Include the current half-pixel box tolerance and one-pixel document-scroll tolerance at and just beyond their boundaries. Assert the failure category and enough of the reason to identify the affected edge or clipping container.

Keep this to the existing behavior, with no browser, screenshots or model calls. It fits a thirty-minute first task. Can you run node --test proof/test/overflow.test.mjs while working, then npm test with the prerequisites in CONTRIBUTING.md?