diff: skip selector-list factoring in the canonical projection

[samoht]

Selector-list factoring is input-shape dependent: whether a shared declaration can be lifted depends on how the input grouped its selectors, and once lifted, an intervening rule writing the same property can make putting it back unsafe. The canonical projection was therefore not confluent - the same stylesheet written factored and written inline mapped to different forms.

In a Tailwind parity run this showed up as .text-xs\/4 losing a declaration:

tw as authored:  .text-xs\/4{font-size:var(--text-xs);line-height:calc(var(--spacing)*4)}
after optimize:  .text-xs,.text-xs\/4,...{font-size:var(--text-xs)}
                 .text-xs\/4{line-height:calc(var(--spacing)*4)}
after canonical: .text-xs\/4{font-size:var(--text-xs)}
                 .text-xs\/4{line-height:calc(var(--spacing)*4)}
reference:       .text-xs\/4{font-size:var(--text-xs);line-height:calc(var(--spacing)*4)}

expand_lists undoes the grouping but cannot restore the merge, because .text-xs sits between the two rules and writes line-height too. The differ then pairs the first rule and reports the second's declaration as missing. Both sides now canonicalise to the reference's single rule.

Css.optimize gains ?factor (default true); only the canonical projection passes false, so fmt --minify still factors.