test: spacing-utilities regression for issue #61

[jenil]

Summary

Validation-only regression test preserving the resolved spacing utilities from
issue #61 as a permanent Chromium VRT-suite check.

Changed files

• test/spacing-utilities.html — minimal fixture with data-test-id locators, no display-only JS
• test/vrt/validations/spacing-utilities.spec.js — 9 computed-style assertions
• package.json — extended test:vrt script (one path addition)

Assertions

#	Assertion	Element
1	baseline .container has 10px horizontal padding	.container
2	.container.is-marginless has all margins 0px	.is-marginless
3	.container.is-paddingless has all padding 0px	.is-paddingless
4	baseline .row has horizontal margins -10px	.row
5	.row.is-marginless has horizontal margins 0px	.is-marginless
6	baseline .col has side/bottom margins 10px	.col
7	.col.is-marginless has all margins 0px	.is-marginless
8	.col-6.is-marginless receives utility override	.col-6.is-marginless
9	nested .container > .row > .col remains correct	full nesting

Validation

npx playwright test test/vrt/validations/spacing-utilities.spec.js
  --project=chromium --reporter=line --output=/private/tmp/chota-136-results
→ 9 passed, exit 0

yarn test:vrt
→ 28 passed (19 existing + 9 new), exit 0

Environment

• OS: Darwin 25.5.0 (macOS)
• Node: v24.16.0 / Yarn: 1.22.22
• Playwright: 1.62.0 (Chromium)
• Viewport: 1280x720 (default)
• Base SHA: 32e0c7e

Disposition

Close #136 — the spacing utilities from #61 are already implemented and
documented. This PR adds regression coverage to prevent future regressions.