Fix trailing line-block spacing in table cells - #1702

Summary

Fixes #1701.

Reset bottom margin only on a line block that is the direct last child of a
table header or data cell. Preserve spacing between blocks and outside tables,
and leave font size and line height unchanged. Add a demo regression covering
headers, stub columns, nested lines and blocks followed by another paragraph.

The checked-in CSS was regenerated with the existing Node 14.21.3,
node-sass 4.14.1 and Webpack 4 toolchain; the lockfile is unchanged.

Validation

• Original assets reproduced the trailing 24px margin in Chrome and WebKit.
• npm ci --no-audit --no-fund, node bin/preinstall.js, npm run build passed
  in an isolated Node 14 container.
• Actual Sphinx 6.2.1, 7.4.7, 8.0.2 and 9.1.0: three package tests per version
  passed and demo documentation built successfully with the new assets.
• Chrome 152 and WebKit 26.5, four Sphinx outputs, widths 1440/768/390:
  24 combinations passed geometry assertions. Final blocks have 0px bottom
  margin; intermediate/outside blocks retain 24px; nested bottom gap is 0px.
• Desktop/mobile screenshots inspected; git diff --check passed.

Firefox and Sphinx development-head were not tested. Existing intentional
invalid-reference demo diagnostics and build-tool deprecation warnings remain.

AI assistance: implemented, tested and reviewed with OpenAI Codex. No claim of
manual human review is implied.