What

Adds a per-product documentation version selector at the top of the sidebar, driven
by _data/products.yml (introduced in #328).

For the current product it lists the versions (newest first), marks the one being
viewed, and badges the version that latest aliases — e.g. 9.x · 8.x (latest).

Design notes

• Per-product, not global. The theme's built-in selector is a single global
  dropdown, which can't express per-product versions on a multi-product site, so
  this is a custom picker reading site.data.products.
• Hidden until there's a choice. A product's picker only renders when it has 2+
  versions, so nothing shows on the site today (every product has one version) — it
  appears automatically at the first cutover. single_version products (OpenVox
  Containers) are always skipped.
• Version-root links. Switching points at each version's root (e.g.
  /openvox/9.x/). The sidebar persists across Turbo navigation, so root links stay
  correct without per-page rewriting. Keeping the reader on the same page across a
  switch (path-preserving) would need per-navigation JS and is a possible follow-up.
• Turbo-aware. syncVersionPicker (in layout_end.html, alongside the existing
  nav-group sync) shows the current product's picker and marks the active version on
  each turbo:frame-load, with a fallback to the latest-aliased version on
  /<product>/latest/ pages.

Validation

Verified against a local two-version test bed (OpenVox 8.x + a 9.0.0-alpha1 9.x),
then torn down:

• Direct loads of /8.x/, /9.x/, and the /latest/ alias: correct active marking
  and latest badge.
• Turbo navigation between products: the picker swaps to the right product and marks
  the active version (browser-tested).
• OpenVox Containers (single_version) excluded.
• With the current single-version state, no picker renders (no stray wrapper).

Part of #325.