Careers CSS section sits outside @layer theme in official stylesheets

Found during prism's QA (#135): every official stylesheet's `@layer theme` closes mid-file (`} /* @layer theme */`) and the careers layout section (plus its media queries) that #96 appended sits after it, unlayered. Consequence: those rules beat site `custom.css` by source order instead of losing by layer rules - the opposite of the documented contract - and anything appended at file end silently lands outside the layer (prism's mesh CSS initially fell into the trailing media query).

Fix: move the careers section inside the layer in all official themes (default, cherrybomb, porcelain, teletype, prism), regen goldens; a gate that asserts the last top-level block of a theme stylesheet is the `@layer theme` block would keep it fixed.

Inspect the official stylesheets for the default, cherrybomb, porcelain, teletype, and prism themes, focusing on the closing @layer theme block and the appended careers section and media queries. Move those sections within the layer, regenerate the goldens, and add the proposed check that the final top-level block is @layer theme.

Written by the indexing model from the issue text.