Light Theme Layering

The light theme uses existing theme tokens to create subtle Material-style tonal layering without introducing a public elevation or surface API.

Token contract

The intended light-theme luminance order is:

1. theme.chrome.rail-background — rail surface, slightly darker/cooler than the graph background.
2. theme.graph.background — quiet cool off-white app/graph baseline.
3. theme.chrome.panel-background — panel chrome, slightly lighter than the app background.
4. theme.card.background — card, dialog, and toolbar surface, slightly lighter than panel chrome.
5. theme.panel-border — mostly opaque outline, visibly darker than panel and card surfaces.

Widgets should continue to consume these through existing resolvers and theme maps: resolve-chrome-background, resolve-card-colors, Card, and existing HUD panel border handling.

Non-goals

Do not add public surface, elevation, shadow, z-depth, or per-widget layering APIs for this behavior. If future work needs those concepts, it should introduce a separate design spec instead of overloading this note.