Initialize ComponentNode.defaultProps lazily (null by default), and
allocate the object only when `props()` is called with defaults.
In the update fast path, skip the `Object.assign({}, props)` copy and
the defaults loop entirely when defaultProps is null — the common case
for components that have willUpdateProps hooks but no prop defaults.

The assemble.mjs script copies doc/v2/.vitepress/dist into the website
output, but the pages workflow never ran build:doc-v2, so the directory
did not exist on the runner and cpSync failed with ENOENT.