feat(cms-sections): variant drag-and-drop reorder + variant-aware preview

[aka-sacci-ccr]

Summary

• Drag-and-drop reorder for variants. Both section variants (SectionVariantList) and page variants (PageVariantTabs) are now reorderable via dnd-kit, with a new reorderMultivariateSectionVariant op and onReorder handlers that follow the moved/selected variant. Order is semantically meaningful (deco renders the first matching variant), so this lets users control precedence.
• Variant-aware preview. Clicking a variant forces the preview iframe to render that exact variant through the deco runtime's x-deco-matchers-override query param. New pure, unit-tested helper variant-matcher-override.ts builds the override ids for inline matchers (dotted resolve-chain path, incl. the .section segment for Lazy-wrapped sections and page-variant nesting) and saved matcher blocks (resolveType).
• Only on explicit click. Entering/auto-selecting a section no longer re-navigates the iframe — the override is applied solely when the user clicks a variant row (or a page-variant tab).
• Scroll preservation. The injected CMS script persists the preview scroll to sessionStorage and restores it (with retries to outlast re-hydration) across the override navigation and the post-save reload, so the preview no longer jumps to the top.

Files

• section-variants.ts (+ test): reorderMultivariateSectionVariant
• section-variant-list.tsx: dnd-kit sortable list
• sections-editor.tsx: reorder handlers, gated syncVariantPreviewOverride, page-variant override wiring
• variant-matcher-override.ts (+ test): override-id/param builders
• sandbox/preview/preview.tsx + cms-editor-script.ts: apply override to iframe src, scroll persistence

Test plan

• Section with ≥2 variants: drag to reorder; order persists after reload; click still selects; duplicate/delete still work.
• Clicking a variant flips the rendered variant in the iframe; entering a section does not reload.
• Page variant tabs: select/reorder updates the preview.
• Scroll down, switch/reorder a variant → preview stays put (no jump to top).
• bun run check, bun run lint, bun run fmt:check, bun test for the touched units.

Made with Cursor

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Summary by cubic

Adds drag‑and‑drop reordering for section and page variants, and makes the preview render the exact variant you click using matcher overrides, with scroll preserved during reloads.

• New Features
  • Reorder section variants and page variant tabs via @dnd-kit/* with a new onReorder handler; selection follows the moved item.
  • New reorderMultivariateSectionVariant op to persist variant order.
  • Clicking a variant forces the iframe to that variant using x-deco-matchers-override; helpers buildSectionVariantOverrideParams, buildPageVariantOverrideParams, and withVariantMatcherOverride (unit‑tested).
  • Overrides trigger only on explicit clicks; entering/auto‑selecting a section no longer navigates the preview.
  • CMS script preserves preview scroll across override navigations and post‑save reloads (sessionStorage + retried restore); iframe reloads avoid focus steal.

^{Written for commit c418a4b. Summary will update on new commits.}