fix(accessibility): make focus appearance contrast-robust

[seonghobae]

Objective

Advance issue #324 by giving keyboard focus a deterministic qualifying indicator on both light and dark surfaces. This is a current-protected-base reconstruction of the unique semantic delta preserved on stale branches fix/focus-appearance-contrast / fix/focus-appearance-main, which are behind protected main and have no open PR.

Test-first state

This Draft starts intentionally RED at exact test-only head 4868211b03dc86627b2c0a716b5b0f91bd00cad1, directly based on protected main 55d7ae8647208e301f282350f076eeddaba61d11.

scripts/test_viewer_focus_appearance.py parses the actual :focus-visible rule, resolves the current limited color syntax, computes WCAG relative-luminance contrast, and requires at least a 2px qualifying band at 3:1 against both white and black backgrounds. Protected main's single modern-browser lightened coral outline is expected to fail the dark/light robustness contract.

Scope

Bounded CSS/accessibility slice under #324. It does not touch viewer.js generation safety (#322/#323), demo.js/dom-utils.js (#264), Java/controller work, or canonical documentation owned by #305.

Acceptance

Observe exact-head RED, apply the smallest two-tone focus rule reconstructed onto current main, then require script tests plus full exact-head CI/Security Scan/SAST/fuzz, zero valid unresolved findings, fresh exact-head/live-base identity, and qualifying independent approval where live protection requires it.

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