Finishes the run-diff view and brings the existing pages onto a shared status palette.

• Adds semantic status color tokens (--success, --warning) to the theme, light and dark, both at WCAG AA contrast for text. ScoreCell, DeltaCell, PassRate, and the diff page now consume them instead of hardcoded text-green-*/text-red-*.
• ScoreCell conveys pass/fail by glyph plus an accessible label, not color alone.
• Run diff: a distinct "no shared cases" empty state (when two runs have no comparable cases, e.g. different dataset versions) with a button to pick another baseline. The "no regressions" banner no longer shows when there are no comparable cases.
• Trend chart: runs on the same calendar day get a time-qualified X-axis label so their points no longer collapse onto one tick.

Solid status pills (StatusBadge) keep their existing colors; tokenizing them would need a separate foreground treatment and is out of scope. Build and eslint clean.