Remove an item from the cart

Removes the line for the `:variantId` variant (its `var_` wire id) from the caller's cart and recomputes the totals, returning the updated cart. A malformed variant id, a variant that is not in the cart, and having no cart at all all return the same 404, deliberately indistinguishable. Removing also SELF-HEALS the cart (Phase 5): any OTHER line whose variant has since stopped being sellable is auto-removed and reported in `removedLines`. A still-addressable target pruned this way returns the healed cart. If the target's variant was hard-deleted, its FK is already NULL and the former id cannot be correlated: the heal persists, but DELETE returns the uniform 404 with no report body, indistinguishable from an absent target. > **Note:** This endpoint requires the e-commerce feature to be enabled on your site.