Bring the development-stages glossary into the new design

[openipc-ai]

Part of #206 (Phase 2). Depends on nothing. Lands before #215, which reuses its badges.

Why

/stages-of-firmware-development is the glossary behind every status badge in the hardware catalogue: six cards (NEQ, R&D, HLP, WIP, MVP, DONE) and a sidebar on how a SoC moves through them. It is linked from five catalogue views (app/views/cameras/socs/_stages.html.erb, show, update, the two "is weird" pages), from the get-started and ecosystem copy in all three locales, from the navbar and from the footer. It still renders in the old container, and the one rule left in pages/_legacy.scss (.col > h3) exists for its sidebar heading. The new design already has components/_stage-badge.scss.

What to change

• content_for :fullwidth; the six cards rebuilt with the stage-badge component and the section components the seven relaunch pages use; the lifecycle sidebar becomes a section rather than a fourth column.
• Copy unchanged; it is translated and correct. The URL is unchanged; the five catalogue links and the two i18n links keep working.
• .col > h3 leaves pages/_legacy.scss, which then has no rules left (the file is deleted in Remove the legacy container and stylesheet once no page needs them #217).
• The page moves from legacy_pages_test.rb to relaunch_pages_test.rb.

Done means

• Renders in three locales, full-bleed, with the badge component; the catalogue links resolve.
• pages/_legacy.scss contains no rules.
• Screenshots before any PR.