fix(openstory): keep centered portal stories stable

[ben-million]

Summary

• center #openstory-root instead of the iframe body
• keep the dev and static-build iframe styles in one shared constant
• add a regression test and patch changeset

Why

Base UI and other portal implementations append a wrapper directly to body. When OpenStory's centered layout made body a grid container, that wrapper became a second grid item and moved the story root. Hover-triggered portals then moved their own trigger out from under the pointer and immediately closed.

Scoping the grid to #openstory-root keeps portaled body children out of the centering layout.

Validation

• nr build
• nr test — 239 passed, 6 skipped
• nr lint
• nr typecheck
• nr format
• nr format:check
• Playwright validation against the million-ui Navigation Menu, Preview Card, and Tooltip stories
• static OpenStory build validation