Expand the reativa (OCaml + Melange) examples into their own package,
@xpecs/reativa — the ReasonML/OCaml sibling of @xpecs/xote — covering the
same set xote does: 22 elements, 8 components, and 7 blocks, written in .mlx
over Reativa.View with the same behaviour and accessibility semantics.

Package (packages/reativa):
- One module per component (Button, Alert, Dialog, Select, StatGrid, …) plus
  shared Ui/Icons/Icon and a generated Contracts.ml (enum prop types derived
  from the specs' `## API` contracts, so the OCaml compiler enforces the
  implementation can't drift from the spec — same guarantee as @xpecs/xote).
- Registry.mlx renders one live example per spec (matching the website's Xote
  examples) and exports the JS surface the site consumes: mount_example,
  example_ids, built.
- scripts/generate-contracts.mjs emits Contracts.ml; scripts/build.mjs clones
  reativa (pinned), compiles the sources inside its dune project with Melange,
  and bundles Registry.js to dist/reativa.bundle.js with esbuild.

Website:
- The example block now has two tab strips: view (Preview / Playground / Code)
  and implementation (Xote / Reativa). The implementation strip swaps which
  library renders the live preview; Code stays the Xote source.
- ReativaExamples reads example_ids from the bundle so the two never drift;
  build-reativa.mjs builds @xpecs/reativa and copies its bundle into the site.
- Removes the old single-file website/reativa proof-of-concept.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TmsZSd9fQyhGdd4JbX7QLp