docs(wix-app): migrate cart events + site-plugin read to Cart V2 (follow-up to #855)

[rommy-amitai-w]

What

Follow-up to #855. Migrates the two remaining Cart V1 references in wix-app to Cart V2:

• references/backend-event/COMMON-EVENTS.md — import { cart } / cart.onCartCreated / cart.onCartUpdated → cartV2.*, with cart-v2 doc links.
• references/SITE_PLUGIN.md — import { currentCart } + currentCart.getCurrentCart() → currentCartV2 + const { cart } = await currentCartV2.getCurrentCart().

Why a separate PR

#855 deliberately scoped wix-app out (it sits outside the storefront ecom skills, and its eval areas — backend-event, site-plugin — are on the separate, soak/non-blocking wix-app eval gate, not the blocking wix-manage one). This PR completes the "no Cart V1 / Checkout V1 in any ecom component" goal for wix-app.

Notes

• The wix-app coverage gate is in its soak period (WIX_APP_EVAL_BLOCK_MERGE unset → blocking:false), so these edits pass with a coverage warning rather than a block. Authoring a build-based wix-app eval scenario for backend-event/site-plugin is left to the wix-app owners (non-blocking).
• The other straggler Ayal flagged — wix-docs-base44/SKILL.md's /ecom/v1/carts/current/add-to-cart example — is already resolved on main: the skill was renamed to wix-base44-connector (rename to wix-base44-connector + scratch under the same roof #1059) and its cart-prose was dropped (polish(wix-base44-connector): /tmp scratch, mgmtRecipes(), fuller recipe list, drop cart-prose #1083). Nothing left to change there.

Docs-only; no code behavior.

[github-actions]

⚠️ EvalForge Skill Gate: Coverage Warning

• backend-event has no eval scenario at all. Add one under yaml/wix-app-evals/ tagged backend-event, or add that tag to a scenario that already exercises the area.
• site-plugin has no eval scenario at all. Add one under yaml/wix-app-evals/ tagged site-plugin, or add that tag to a scenario that already exercises the area.

No eval run was started — a coverage failure is caught before any run cost.

The gate is in its soak period (blocking: false), so this check still passes.