fix: report settled canvas zoom through latest callback

[steve8708]

Summary

• keep the latest onZoomChange callback for debounced wheel settle
• preserve the existing Design E2E host readout regression coverage

Validation

• pnpm exec oxfmt --check templates/design/app/components/design/MultiScreenCanvas.tsx templates/design/app/components/design/MultiScreenCanvas.wheel.test.tsx
• pnpm --filter design exec vitest run app/components/design/MultiScreenCanvas.wheel.test.tsx

[builder-io-integration]

Builder reviewed your changes and has a few items to flag 🟡

Review Details

Code Review Summary

PR #5520 makes a focused React lifecycle correction in MultiScreenCanvas: the debounced wheel/pinch-settle callback now reads the current onZoomChange through a ref, so a Design Editor re-render during a pending gesture cannot route the settled zoom report to an obsolete callback. Removing the callback from the settle effect dependencies is consistent with that ref-based design, and the surrounding cleanup remains intact.

Risk assessment: Standard (shared UI state/interaction behavior). Two independent code-review passes found no confirmed runtime, cleanup, or stale-closure bugs. One reviewer identified a low-severity coverage gap: the existing wheel test does not replace onZoomChange while the debounce is pending, so the exact regression scenario is not directly protected by a test.

🧪 Browser testing: Attempted FULL verification. The dev server was healthy, but all planned browser cases were couldnt_verify because the executor environment exposed no Chrome/browser automation tools.