feat: add stable React node keys

[tiye]

Summary

• add VirtualNode::with_key for elements, components, fragments, and text
• key TodoMVC list entries by stable todo IDs
• fail browser regressions on React console warnings and errors
• document the reconciliation-key API and update the generated interface

Verification

• moon check --target js
• moon test (38 passed)
• moon info && moon fmt
• corepack yarn build
• real-browser TodoMVC flow covering editing, creation, toggle-all, filtering, and clearing; no browser warnings or errors

Relates to #7.

[Copilot]

Pull request overview

This PR introduces a stable “key” API for MoonBit VirtualNodes so React can reconcile dynamic collections without warnings, then applies it to the TodoMVC example and tightens browser regression checks to fail on React console warnings/errors.

Changes:

• Added VirtualNode::with_key plus the needed JS interop to assign reconciliation keys across elements, components, fragments, and text.
• Keyed TodoMVC list items by stable todo IDs to avoid index-based reconciliation.
• Made the Playwright TodoMVC regression fail on browser console warnings/errors, and documented the new API (README/changelog) while updating the generated interface.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/todomvc.spec.mjs	Captures console warnings/errors during the TodoMVC flow and fails the test if any occur.
src/react.mbt	Adds VirtualNode::with_key and JS interop (cloneElement) to support stable keys across node kinds.
src/react_test.mbt	Extends the test React stub with cloneElement and adds a regression test for with_key.
src/pkg.generated.mbti	Exposes VirtualNode::with_key in the generated public interface.
src/main/todolist.mbt	Applies stable keys to rendered todo items using todo.id.
README.md	Documents recommended with_key usage for dynamic collections.
index.html	Adds a data URL favicon to avoid console noise (e.g., favicon 404) in browser regressions.
CHANGELOG.md	Notes the new with_key API and stricter browser regression behavior.

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