Fix callout glyphs in forced colors and correct prototype guidance

[jonathanKingston]

In forced-colors mode, the five callout mask backgrounds introduced by #2404 were repainted to the same color as their plate, hiding the glyphs. Use system CanvasText and disable automatic color adjustment only on the glyph pseudo-element, leaving the rest of each callout responsive to the user's palette.

Add a regression that fails on the previous implementation and passes in both dark and light forced-colors palettes. Cover reduced-transparency and increased-contrast hatch fallbacks, and include five focused screenshots for the forced-colors callouts and expanded Thinking states.

Correct the prototype and UI guidance to describe the actual numeric tuning tokens, per-component severity mixing, standing-proposal rail exception and VNC gutter sizes. Remove the incorrect claim that an 8px diagonal hatch guarantees freedom from aliasing. The prototype remains an archive; a living guide built from production components is a separate follow-up, as are custom-accent collisions and exact VNC gutter alignment.

Validation

• All pnpm run check stages passed before rebasing onto main: typecheck, lint, formatting, demo sync, dead-code, oracle, e2e syntax and 8,751 unit tests. One formatting failure was corrected, then the remaining stages were resumed.
• Build, all five callout tests and all six required markdown specs passed. Initial renderer setup timeouts passed on retry.
• The new forced-colors regression failed against the unfixed implementation, then passed in both palettes. The five new screenshot fixtures were visually inspected.
• Rebased onto main after Ship the side-highlight prototype: a rail means nesting now #2404 merged. The fresh build and all five callout tests passed (the first run timed out during setup; retry passed). Full CI is running for the follow-up.

[copse-release-bot]

Reference screenshots ready for review

Review GitHub’s image diffs in screenshot PR #2612.
Merge it (or enable auto-merge) to apply the accepted PNGs to codex/callout-accessibility-fix.

Rendered for ab4340287a7b by CI run 34302749558; the immutable artifact is reference-screenshot-candidates-34302749558.

If this source branch moves, a later successful render closes the stale review PR and replaces this link.