6.0.1 — sidebar corrections, and one stylesheet for the site

Fixes to what 6.0.0 shipped, plus the end of the site's duplicated CSS.

Fixed

• The sidebar panel stretched to the full viewport, so a short contents list sat at the top of a tall empty box and a long one scrolled inside a panel that gave no sign it could. It is content-height now, capped at the viewport.
• A panel that does scroll draws a Win9x scrollbar — square track, bevelled thumb, both from tokens — instead of the platform's thin modern overlay bar.
• A nav or list inside the sidebar drew its own frame inside the panel's frame inside the column. The panel is the frame; its contents are separated by hairlines and by group bands bled to the panel's edges.
• Sidebar rows were body-sized with full nav padding, so a forty-link contents ran 1500px. They are list rows now.
• The title bar stays pinned while the panel scrolls, as a window's does.

Site

• The home page, the documentation and all eight examples share one site.css and carry no inline style block. They had a copy each of the same page CSS, which had drifted — the home page ran full-bleed while the docs were centred at 1320px, the footer was duplicated seven times and the whole sign-in layout twice. Every page now uses the same shell, --retro-container-xxl.

All five gates pass: 36 unit tests, 370 token pairs at AA, the stylesheet gate, 140 rendered-page checks, 20 keyboard loads.