Second Codex stop-hook caught: the previous fix only guarded fill-level
opacity (`fill.opacity === 0` / 8-hex alpha 00). PenNodeBase has its
own `opacity?: number | string` and `visible?: boolean` fields that
hide the WHOLE wrapper including its fill. A wrapper with
{ fill: [{type:'solid', color:'#FFFFFF'}], opacity: 0, ... }
was still being treated as a white bg and masking the real bg further
up the chain.
ancestorBgColor() now skips ancestors whose node-level
`opacity === 0` or `visible === false`, complementing the
firstSolidColor fill-level guard. `opacity` can be a `$variable` ref
in PenDocument; resolving that to a literal 0 is not yet covered —
we only catch the literal-0 case for now (which is the AI-output
shape the corpus produces).
Two new test cases cover both paths.
8ddefec481