Add a real, spec-compliant CSS3/4 engine (ported from ExCSS via PeachPDF)

[jhaygood86]

Summary

This replaces HTML-Renderer's ad-hoc CSS parsing/matching with a real CSS3/4 spec-compliant CSS engine — a proper cascade, a real selector engine (combinators, attribute selectors, structural pseudo-classes, :not()/:is()/:has(), ::before/::after), @media condition parsing, and calc() — in place of the old simple class-name-keyed block lookup.

The engine is derived from ExCSS, sourced from the more up-to-date fork maintained in PeachPDF, and ported into Source/HtmlRenderer/Core/CssEngine. Two of HTML-Renderer's own non-standard CSS extensions — corner-radius and background-gradient — depended on the old parser and have been replaced with their spec-compliant equivalents (border-radius, linear-gradient()) so the demo app keeps working against the new engine.

Details

• CSS engine: Ported from ExCSS, but sourced from the more up-to-date version maintained by PeachPDF, rather than upstream ExCSS.
• Minimal CSS fixes: Applied only what was needed to keep existing functionality working — e.g. replacing the corner-radius and background-gradient extensions with spec-compliant versions (see below).
• Scope: This is a minimal port of the engine and the fixes needed to resolve the regressions it introduced. It is not a full CSS3/4 compliance pass — follow-up PRs will incrementally improve spec compliance on top of this engine.

What now works end-to-end

• Real cascade: origin-aware (user-agent vs. author), specificity- and document-order-ranked, with a correct !important tier and the CSS-wide keywords initial / inherit / unset / revert / revert-layer.
• Full selector engine: combinators (descendant, >, +, ~), comma-separated selector lists, attribute selectors ([attr], [attr=v], [attr~=v], [attr*=v], [attr^=v], [attr$=v], [attr|=v]), :not(), :is()/:matches(), :has() (descendant form), structural pseudo-classes :nth-child()/:nth-last-child()/:nth-of-type()/:nth-last-of-type() (including the CSS4 An+B of <selector> form) and :nth-column()/:nth-last-column(), :first-child/:last-child/:first-of-type/:last-of-type, :only-child/:only-of-type, :root, :link, and ::before/::after generated-content pseudo-elements.
• @media conditions: parsed, indexed, and matched, including nesting and not/type conditions.
• calc(): parsed and evaluated for property values.
• content:: supports attr() and the open/close quote keywords.
• Spec-compliant background-image: linear-gradient(...), replacing the old proprietary background-gradient extension.
• Spec-compliant border-radius, with independent per-corner horizontal/vertical elliptical radii, replacing the old proprietary corner-radius extension.
• Rule indexing: rules are bucketed by tag/class/id so cascade cost no longer scales as rules × boxes on large documents.

Known limitations (not yet wired up)

• @media conditions are parsed and indexed, but this engine has no live "what device are we rendering for" concept, so cascading is always done against media all — non-all conditions (e.g. @media screen) never actually gate anything. This matches the old engine's pre-existing behavior; it's not a regression.
• radial-gradient() parses but isn't painted yet — only linear-gradient() renders.
• ::marker wasn't ported; list markers are still painted procedurally rather than as a generated box.
• content: doesn't support counter() / counters() / string() / content().
• :has() only supports the default descendant relative-selector form (no leading >, +, ~).

Related open issues

Confirmed by reading the relevant code paths (not just title-matching):

• corner-radius vs border-radius #152 — corner-radius vs border-radius: fixed directly (see above).
• CSS selector for wildcard classes does not work #169 — wildcard attribute class selectors ([class*="language-"]) don't match: fixed — the new engine has real attribute-selector support.
• css class names with periods don't work #168 — compound multi-class selectors (.token.property) don't match: fixed — the old engine's single-class-key lookup couldn't represent this; the new selector engine requires every class in a compound selector to match.
• ParseCssBlockProperties does not parse url attribute correctly #29 — url('data:...;base64,...') mis-parsed because of the semicolon inside the quoted string: fixed — the old ad-hoc parsing method is gone, replaced by a real, spec-correct tokenizer.
• ParseBorderProperty fails on default included CSS #28 — border: 1px inset; mis-parsed: fixed — the old ad-hoc border-shorthand parser is gone, replaced by a real, order-independent border shorthand implementation.
• Super slow performance when there are lots of "a" html tags #239 — severe slowdown with many <a> tags: likely resolved or significantly improved — the issue's own root cause (an O(rules × boxes) matching function with a hand-rolled <a>-tag special case) has been replaced by the new tag/class/id rule index. Not independently benchmarked.
• css background-color is not working #138 — background: shorthand set via a descendant selector not applying: likely fixed — both prerequisites (real descendant-combinator matching, real background shorthand parsing) are now in place, though the exact reported case wasn't directly reproduced.

Not addressed by this PR (layout/painting gaps, not parsing/cascade): #203 (position: absolute), #34 (float), #42 (table border-conflict resolution), #104/#103 (<tr> display/background-color).