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

[bennypowers]

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

Source	Element	Status
PFE v4	--	--
cem	<pf-v6-page>	Exists
cockpit	<pf-v6-page>	Exists
chickadee	--	--

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)