[feat]: Create `<pf-v6-page>` element

Start with the PatternFly v6 Page reference and .claude/ADVICE.md, then review the listed Page, PageSidebar, and PageSection APIs and slot requirements. Done means the web components provide the requested responsive layout and API, with demos, public API and a11ySnapshot tests, review checks, a changeset, and a PR targeting staging/pfv6.

Summary

Create a <pf-v6-page> web component implementing the PatternFly v6 Page component.

Top-level application layout providing masthead, sidebar, and main content areas with responsive behavior.

Prior Art

Sub-components

• <pf-v6-page-sidebar> - Collapsible sidebar container
• <pf-v6-page-section> - Content section within the main area

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with @patternfly/react-core Page
• MAY adjust element API to leverage web platform strengths
• SHOULD maintain CSS custom property theming compatibility
• SHOULD expose useful CSS shadow parts

Element-specific considerations

• isManagedSidebar prop: page controls sidebar open/close state
• isNavOpen / defaultNavOpen for sidebar visibility
• mainContainerId for skip-to-content link target (coordinate with pf-v6-skip-to-content)
• Named slots: masthead, sidebar, breadcrumb, notification-drawer, default (main content)
• PageSection: variant (default/light/dark/darker), padding, isFilled, isWidthLimited
• PageSidebar: isSidebarOpen, theme (light/dark)
• Breadcrumb and notification drawer slot integration
• Main area <main> element should have id matching mainContainerId

Checklist

Implementation

• API per .claude/ADVICE.md
• CSS uses v6 tokens, logical properties, nesting
• @summary and JSDoc on element class

Demos

• Demos match patternfly.org naming/structure
• CSS custom properties match computed styles from patternfly.org (Chrome MCP)
• Visual parity verified via Chrome MCP

Tests

• Public API covered (attributes, properties, events, slots)
• a11ySnapshot assertions

Reviews

• /review-api passes
• /review-demos passes
• /review-a11y passes
• eslint and stylelint pass

Ship

• Add changeset
• PR targets staging/pfv6 (NOT main)