- Add sticky_contents config option (defaults to False)
- Use position: fixed for reliable sticky behavior
- Add scrollspy.js module for tracking scroll position
- Highlight active section in TOC as user scrolls
- Support scrollable TOC for long content lists

Fixes #133

Usage:
  html_theme_options:
    sticky_contents: true

- Shows floating button at bottom right when page is scrolled down
- Only appears when sticky_contents is enabled
- Uses modern Sass color.adjust() function
- Animated appearance with fade and slide transition
- Button hidden until user scrolls past 300px threshold

- Subsections are hidden by default in sticky TOC mode
- When a section becomes active, its parent sections expand
- When scrolling away, subsections collapse back
- Smooth animated transitions for expand/collapse
- Only affects sticky mode - non-sticky TOC unchanged

- Subsections now stay expanded while scrolling through content
- Parent section stays expanded when any child is active
- Also expands parent's children when parent heading is active
- Only collapses when scrolling to a different top-level section

- Track which top-level TOC section each item belongs to
- When active item changes, keep its entire top-level section expanded
- Expand ALL nested items within the active top-level section
- Subsections only collapse when scrolling to a different top-level section
- Uses :has() selector for finding items with children

- Fix parent traversal to correctly find top-level TOC item
- Replace :has() selector with :scope > ul for broader browser support
- Ensure all nested items with children get expanded class

- New theme option: contents_autoexpand (default: false)
- When false: TOC shows all sections normally (no collapse/expand)
- When true: Subsections auto-collapse/expand based on scroll position
- Allows sticky TOC to ship while auto-expand is refined
- Uses data-autoexpand attribute to pass config to JavaScript

Usage in _config.yml:
  html_theme_options:
    sticky_contents: true
    contents_autoexpand: true  # optional, defaults to false

- Start from parent ul of active item, not the item itself
- Walk up ul->li->ul->li chain correctly
- Ensures parent sections get expanded class when in subsection

- Theme config values come as strings ('True'/'False') not booleans
- Updated conditionals to check for both boolean and string values
- Fixes autoexpand not being enabled despite default True in theme.conf

- Change overflow-y from 'auto' to 'hidden' for cleaner appearance
- Content that exceeds viewport height will be clipped at bottom
- Also adds Core Rules section to copilot-instructions.md

- When contents_autoexpand is false, highlight the top-level parent section
  instead of the hidden subsection as user scrolls through content
- Move back-to-top button outside .inner container to prevent clipping
  from overflow-y: hidden

Changed from a sibling <button> after the <a> to an inline <span>
inside the <a>, so the icon flows naturally at the end of the title
rather than appearing on a separate line.

Position the icon absolutely 18px right of the <a> link, vertically
centered. This prevents the icon from taking up inline space and
causing text to wrap to the next line on longer TOC entries.

The absolute positioning pushed the icon outside the 200px container
with overflow:hidden, making it invisible. Instead, use display:
inline-block with width:0 and overflow:visible — the icon visually
appears after the text via margin-left on the SVG, but takes zero
width in the flow so it can never cause a line break.