rule_order: drop shadowed declarations in one backward pass - #218
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For every declaration of every rule the shadow check rescanned the whole tail, serialising each later rule's selector again to compare it. Walking backwards and carrying a per-selector set of the properties written later answers the same question with a lookup.
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canonicalize_rule_orderon a 660KB stylesheet went from 2.90s to 0.62s, with byte-identical output.drop_shadowed_declarationsasked, for every declaration of every rule, whether any later rule with the same selector writes that property, and answered by rescanning the whole tail and serialising each later rule's selector again. Walking the list backwards and carrying a per-selector set of the properties written later answers the same question with a lookup.This is a regression against 1.0.0 rather than a long-standing cost: the function arrived in 923d09f, after the release. On the same input 1.0.0 takes 22.5s end to end and current main 34.5s; this branch is 24.0s while doing strictly more work in the projection than 1.0.0 did.
Stacked on #217; review that first.