Expose `leanVariants` on productListingPage / productListFull loaders

[JonasJesus42]

apps-start: expose leanVariants on listing/shelf loaders

Repo: decocms/blocks (monorepo; VTEX code ships as @decocms/apps-vtex)

Problem

productListingPage and productListFull (shelf) transform every product with
FULL variants, producing multi-MB hydration payloads on PLPs and product shelves.

• src/loaders/intelligentSearch/productListingPage.ts:450
  → toProduct(p, sku, 0, { baseUrl, priceCurrency: "BRL" }) — no leanVariants
• src/loaders/productListFull.ts:147
  → toProduct(p, preferredSku, 0, { baseUrl, priceCurrency: "BRL" }) — no leanVariants

The lean transform already exists (src/utils/transform.ts: toProductVariant,
ProductOptions.leanVariants, variantIncludeImage, variantIncludeInventory)
but ONLY productDetailsPage.ts opts in — the two loaders that generate the
biggest payloads don't.

Impact (miess-01-tanstack, /linha-noite, 36 products)

Each product shipped ~24 SKUs each with: nested isVariantOf.hasVariant (N²),
full priceSpecification installment ladder (~27 entries/SKU), full image
gallery, full additionalProperty. Human PLP HTML = 14 MB; PDP = 9.4 MB.
Worked around at site level with a trimProductForShelf util (lean variants) →
PLP 2.84 MB, PDP 4 MB. That util should not need to exist.

Proposed fix

Support leanVariants on productListingPage/productListFull (a Props flag,
or default-on for listing/shelf contexts). Reuse the existing toProductVariant.

Opt-in requirement

Must not regress sites whose ProductCards read full variant data (color/size
swatches from additionalProperty, per-SKU images). Ship as opt-in (prop, or a
default that sites can disable) — do not silently change existing behavior.