docs site: flat SVG diagrams are low-contrast in dark mode

[jeremi]

Every content diagram on the docs site hardcodes the light palette (#161616/#3a3a3a text, #91 accents) with a transparent background, while src/styles/custom.css gives .sl-markdown-content img background: var(--registry-surface), which is #1b1e24 under [data-theme='dark']. Dark text on a near-black surface makes all 13 in-use diagrams hard to read in dark mode. The SVG a11y gate (scripts/check-svg-a11y.mjs) checks title/desc/role but not contrast, so this passes CI.

Options: give diagram images a fixed light surface in dark mode (e.g. background: #fff for img[src$=".svg"] under the images path), add prefers-color-scheme/data-theme aware styling inside the SVGs, or ship dark variants. First option is the cheapest and matches how many docs sites handle light-palette diagrams.

Found 2026-07-07 during the docs 1.0 diagram review (two independent reviewers flagged it).

[jeremi]

Triage decision for 1.0: required accessibility fix.

Thirteen published diagrams are unreadable in the site's supported dark theme. This is a user-visible documentation defect with a small, low-maintenance repair. Use one fixed light image surface for the existing light-palette SVG assets and add a focused contrast regression. Do not create and maintain parallel dark variants unless the simple surface treatment fails accessibility review.