Implement :nth-child(An+B of S)

[nicoburns]

Summary

Enables :nth-child(An+B of S) / :nth-last-child(An+B of S) parsing and adds the embedder-side invalidation it requires. Matching is already generic in stylo/selectors; the missing piece is that a mutation on one child can change which siblings match (their nth-of index changes), which stylo's downward snapshot invalidation can't reach. This mirrors Gecko's RestyleManager::RestyleSiblingsForNthOf.

Depends on (and temporarily rev-pins stylo to) DioxusLabs/stylo#16, which adds a separate layout.css.nth-child-of.enabled pref (and a layout.css.has-selector.enabled pref for later :has() work). Blitz enables only the nth-of pref. TODO comment notes reverting to crates.io once a stylo release ships the prefs.

Invalidation

New BaseDocument::restyle_siblings_for_nth_of(node_id):

• early-out unless the parent carries HAS_SLOW_SELECTOR_NTH_OF (set by stylo via apply_selector_flags during matching)
• restyles earlier siblings only if parent has HAS_SLOW_SELECTOR (:nth-last-child(.. of S)), later siblings only if HAS_SLOW_SELECTOR_LATER_SIBLINGS (:nth-child(.. of S)), by inserting RestyleHint::restyle_subtree() and marking ancestors dirty

Hooked at the two mutation classes that can change of S matching:

• DocumentMutator::{set_attribute, clear_attribute} — gated by new attribute_might_affect_nth_of, which consults the stylist's nth-of dependency maps (might_have_nth_of_{attribute,id,class}_dependency) using both old and new id/class values
• BaseDocument::snapshot_node_and (element state changes, e.g. :checked) — gated by has_nth_of_state_dependency

Also fixes DocumentMutator::remove_node to insert restyle_subtree() on the parent like the other removal paths already do (plain positional :nth-child needs this too; found via the new insertion/removal test).

Tests

packages/blitz-test-harness/tests/nth_child_of.rs: static matching plus dynamic invalidation for sibling class add/remove, id change, arbitrary attribute change, :checked state change, insertion/removal, and :nth-last-child(.. of S).

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/65287500bfb04b2589b9fc6a08bc2d14
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/65287500bfb04b2589b9fc6a08bc2d14?variant=devin-insiders
Requested by: @nicoburns

WPT results

36 newly passing, 2 newly failing (net +34).

Full diff (38 changed tests)

+ Fail => Pass css/css-color-adjust/rendering/dark-color-scheme/color-scheme-rule-cache.html
- Pass => Fail css/css-color-adjust/rendering/dark-color-scheme/color-scheme-table-border-currentcolor-responsive.html
- Pass => Fail css/css-images/gradient/color-scheme-dependent-color-stops.html
+ Fail => Pass css/css-properties-values-api/registered-property-computation-color-001.html
+ Fail => Pass css/css-properties-values-api/registered-property-computation-color-002.html
+ Fail => Pass css/css-variables/registered-property-light-dark.html
+ Fail => Pass css/selectors/invalidation/nth-child-of-attr-largedom.html
+ Fail => Pass css/selectors/nth-child-and-nth-last-child.html
+ Fail => Pass css/selectors/nth-child-of-attribute.html
+ Fail => Pass css/selectors/nth-child-of-classname-002.html
+ Fail => Pass css/selectors/nth-child-of-classname.html
+ Fail => Pass css/selectors/nth-child-of-complex-selector-many-children.html
+ Fail => Pass css/selectors/nth-child-of-complex-selector.html
+ Fail => Pass css/selectors/nth-child-of-compound-selector.html
+ Fail => Pass css/selectors/nth-child-of-nesting.html
+ Fail => Pass css/selectors/nth-child-of-no-space-after-of.html
+ Fail => Pass css/selectors/nth-child-of-not.html
+ Fail => Pass css/selectors/nth-child-of-nth-child.html
+ Fail => Pass css/selectors/nth-child-of-pseudo.html
+ Fail => Pass css/selectors/nth-child-of-tagname.html
+ Fail => Pass css/selectors/nth-child-of-universal-selector.html
+ Fail => Pass css/selectors/nth-child-specificity-1.html
+ Fail => Pass css/selectors/nth-child-specificity-2.html
+ Fail => Pass css/selectors/nth-child-specificity-3.html
+ Fail => Pass css/selectors/nth-child-specificity-4.html
+ Fail => Pass css/selectors/nth-last-child-of-classname.html
+ Fail => Pass css/selectors/nth-last-child-of-complex-selector.html
+ Fail => Pass css/selectors/nth-last-child-of-compound-selector.html
+ Fail => Pass css/selectors/nth-last-child-of-nesting.html
+ Fail => Pass css/selectors/nth-last-child-of-no-space-after-of.html
+ Fail => Pass css/selectors/nth-last-child-of-style-sharing-1.html
+ Fail => Pass css/selectors/nth-last-child-of-style-sharing-2.html
+ Fail => Pass css/selectors/nth-last-child-of-tagname.html
+ Fail => Pass css/selectors/nth-last-child-specificity-1.html
+ Fail => Pass css/selectors/nth-last-child-specificity-2.html
+ Fail => Pass css/selectors/nth-last-child-specificity-3.html
+ Fail => Pass css/selectors/nth-last-child-specificity-4.html
+ Fail => Pass css/selectors/nth-of-selector-before.html

_{Generated by the WPT workflow.}

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