fix(css): scope pseudo-only selectors, nesting, and deep functions - #82

Summary

Follow-up to #81 (the CSS feature-support matrix). This PR fixes the gaps documented there and moves every it.failing case into supported assertions.

Fixes

• Pseudo-only selectors now receive the scope attribute:
  • :hover → :hover[hash]
  • :is() / :where() / :not() / :has() → …[hash]
  • ::before / ::slotted() → [hash]::before (prepended, since an attribute cannot follow a pseudo-element)
• Leading pseudo-only compounds in complex selectors are scoped (:hover > .b, :is(.a) .b, :not(.a) ~ .b, :where(.a) .b).
• CSS nesting is scoped, both explicit and implicit, including inside at-rules:
  • explicit & rules are found by recursing into rule blocks
  • implicit nesting is parsed as Raw by css-tree, so each raw nested rule is re-parsed, scoped, and written back
• Column combinator ||: css-tree serializes the prelude as Raw; it is now split on top-level ||, each part is scoped, and the parts are re-joined.
• :deep() / :v-deep() function form is supported. A single argument is spliced in place; selector-list arguments become :is(...) with the same matching semantics.

Behaviour notes

• :root and :scope remain global. The root element never receives the scope attribute, so :root[hash] would only turn working rules into dead ones. Descendant subjects are still scoped (:root .b → :root .b[hash]).
• & itself does not receive a second attribute — the parent rule is already scoped.

Tests

src/css/css-features.spec.ts now contains 126 supported cases, up from 92 supported + 19 it.failing gaps.

Verification:

npm test                      # 219 passed
npm run build                 # clean
npm run lint                  # clean
npm run test:e2e              # vite 5/6/7/8, dev + build all pass

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