Codex stop-hook review caught: the detectTextBgContrast ancestor walk
treated any wrapper with a solid `fill` entry as the bg color, even
when the fill was effectively invisible. The classic miss case:
page { fill: cream }
└─ wrapper { fill: [{ type: 'solid', color: '#FFFFFF', opacity: 0 }] }
└─ text { fill: cream }
Without the guard, the detector picked the wrapper's white fill as bg
and reported a healthy contrast ratio against the cream text — masking
the real cream-on-cream failure that lives one level up.
firstSolidColor() now skips fills with `opacity === 0` and 8-hex colors
whose alpha byte is `00` (e.g. `#FFFFFF00`). Both produce no visible
color, so the ancestor walk continues past them to the real bg.
Semi-transparent fills (opacity 0.5, 8-hex alpha 80, etc.) are out of
scope — the detector still treats them as opaque rather than trying to
math the layered composite. Tests pin both: opacity=0.5 + alpha=80
stay treated as bg.
4 new test cases cover the fix plus the boundary (opacity=0.5, alpha=80
should NOT be skipped). Full corpus replay shows 14 hits unchanged on
the 470-row corpus — no false-positive regression introduced.
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