fix(docs): make sidebar navigation scrollable

[AMillionDriver]

Summary

• keep the AX sidebar control pinned while documentation items scroll independently
• keep the active documentation item visible when a page loads
• swap the close X for a panel-close icon on hover and keyboard focus
• add docs contract checks for the scroll region and close-icon state

Root Cause

The sidebar container itself handled overflow, so long navigation lists could extend behind the Windows taskbar. A legacy mobile .is-open { display: block; } rule also overrode the constrained flex layout when the sidebar expanded.

Testing

• Prettier check
• Documentation syntax and catalog check
• Root ESLint
• Root Vite build
• Axoloth Style and Behavior API contract checks
• 33 accessibility tests
• 44 visual regression tests
• Axoloth IntelliSense compile and tests
• Manual 900x600 viewport test in collapsed and expanded states
• Verified the final navigation item is reachable and the sidebar header remains pinned
• Verified no browser console errors

The Vite build retains the existing non-blocking warning for a JavaScript chunk larger than 500 kB.

[AMillionDriver]

Cool