Replace flat InvalidationSet with recursive InvalidationPlan trees
that preserve selector combinator structure. Previously, selectors
with sibling combinators (+ and ~) fell back to whole-subtree
invalidation. Now the StyleInvalidator walks the DOM following
combinator-specific rules, so ".a + .b" only invalidates the
adjacent sibling matching ".b" rather than the entire subtree.
Plans are compiled at stylesheet parse time by walking selector
compounds right-to-left. For ".a .b + .c":
```
[.c]: plan = { invalidate_self }
register: "c" → plan
[.b]: wrap("+", righthand)
plan = { sibling_rules: [match ".c", adjacent, {self}] }
register: "b" → plan
[.a]: wrap(" ", righthand)
plan = { descendant_rules: [match ".b", <sibling plan>] }
register: "a" → plan
```
Changing class "a" produces a plan that walks descendants for ".b",
checks ".b"'s adjacent sibling for ".c", and invalidates only that
element.
