## 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.