Per-major versioning, version switcher, dev page

Reorganised the source so each major version is its own folder, added a version control to the header, and moved the release log to its own page.

• Folder per major version. Site source now lives under src/versions/<major>/ (currently v0), with cross-cutting code in src/shared/. Each major builds to its own self-contained <major>/index.html; the latest is copied to the root index.html.
• Version shown at the top, with a switcher top-right. Picking a version loads that major's build in place (a same-window document replace, so it works in the App host, the preview, and the static domain). The list is driven by versions.json.
• Dev moved to dev.html. The in-page Dev section is gone; the release log is now a standalone page, linked from the header and footer. It still renders straight from dev/releases/.
• The build now produces multiple pages and gates each one independently (syntax, contract scan, version lockstep), then runs the unit suite.