Split the selector engine and the CSS value layer into their own modules

[brainkim]

Two cuts along slot ownership, so no symbol crosses a module boundary, and a sweep so CSS text is parsed in one place.

src/internal/selectors.ts (2155 lines out of dom.ts). Selector parsing, compilation and matching. It reads the tree through four link functions dom.ts now exports, getParentNode, getFirstChildNode, getNextSiblingNode and getPreviousSiblingNode, and the state predicates that already existed as functions, now exported. Those predicates read slots, so they stay in dom.ts; the export list is the selector engine's state interface. dom.ts imports the five query functions back.

src/internal/cssvalues.ts (about 5800 lines out of cssom.ts). Every declaration that touches no slot, no class and nothing from dom.ts: the value grammar, serialization, colors, shorthands, the property tables, and the media query, grid track, easing, calc and counter parsers, with the loops that fill their tables. cssom.ts keeps the CSSOM objects, the cascade, and computed and used values at about 7500 lines, and imports the value functions and tables it needs. The dependency runs one way. Six tree-walking helpers that passed the purity test on a technicality went back to cssom.ts.

One parser. dom.ts parsed IntersectionObserver's rootMargin with its own regex; layout.ts read flex numbers, order, z-index, alignment qualifiers and grid-auto-flow inline; painter.ts parsed font-weight, text-decoration-line, outline-width and the Canvas and Highlight system colors itself. Each now calls a named function in cssvalues.ts.

Namespaces. The six namespace URIs live once in htmltables.ts, beside the tables that name elements in them, instead of three copies.

dom.ts goes from 33,501 to 31,416 lines and cssom.ts from 13,200 to about 7500.

Typecheck, lint, suite 1619/1619 on node and bun, 37/37 examples, lib.dom drift 148 exact.

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