Computing actual WCAG ratios surfaced two real accessibility defects in
the shipped v0.2.0 light theme: tok_string (4.42:1) and tok_flag
(3.15:1) fell below the 4.5:1 the scheme promises for normal text
(NFR-5). Darken both to clear it with headroom (5.18 / 5.98).
Also widen two dark-theme token pairs that were perceptually close
despite distinct hex (hard to separate on good screens): tok_type and
tok_function move from ΔE2000 ~14 to ~18-19 vs their neighbours.
Add four gated tests in theme.rs:
- all_text_colours_meet_wcag_aa_contrast: every text fg >= 4.5:1 on bg,
both themes (the regression gate that would have caught this).
- advanced_mode_border_meets_ui_contrast: 3:1 for the mode-alert border;
plain border is decorative chrome, exempt.
- delta_e_2000_matches_reference_vectors: validates the CIEDE2000 metric
against Sharma et al. reference data.
- syntax_token_colours_are_perceptually_distinct: ΔE2000 >= 15 for every
token-class pair, both themes.
Persist scripts/palette-preview.py: the true-colour swatch + contrast +
ΔE2000 audit tool used to drive these changes (reads the live palette
from theme.rs), for future palette work.
Tests: 2513 passed / 0 failed / 1 ignored (was 2509; +4). clippy and fmt
clean. Relates to NFR-5/NFR-7 (ADR-0057).
