v0.24.0 — index sidebar: accessibility, design tokens, sticky Contracts header

A Layer-3 refinement pass (driven by a design audit). This release also rolls up the v0.23.0 changes, which were committed but never published to PyPI.

New in v0.24.0

• Accessibility: the faint tertiary text (contract source paths, the navigator's R/N visited · U unresolved line, scope and legend text) is darkened to clear WCAG AA contrast in both light and dark themes.
• Design tokens: added --text-*, --space-* (4px base), and --radius-* scales next to the colour tokens; removed every blurry half-pixel font size.
• Contracts navigator: promoted from a small eyebrow label to a sticky section header (list icon + right-aligned count) that anchors the scrolling sidebar; contract names now claim the full row width before ellipsising, with a hover tooltip for long names.
• Markup / a11y fix: the per-contract collapse toggle is now a dedicated <button> (the chevron) sitting beside the pin link, instead of a heading that wrapped the pin (valid HTML; the pin is independently focusable).
• Interface Bindings: the card title stays on one line in the fixed-width sidebar.

Rolled up from v0.23.0 (not previously on PyPI)

• Index terminology refresh: No modifiers / With modifiers / Unresolved facet pills; visited (was reviewed); Interface Bindings card heading.
• Flow-page node rings: a warm ring on the entry/root node, a dashed blue ring on the active (viewport-centre) node.
• solFlow header wordmark.

Pure Layer 3 — no analysis, backend, or dependency changes. Update with `pipx upgrade solflow`.