js+css: implement HTMLInputElement.indeterminate and :indeterminate

[tannevaled]

Summary

• Unlike checked, indeterminate has NO backing content attribute at all (HTML Standard §4.10.5.1.19) — pure script-set runtime state.
• Added dom.Node.Indeterminate (the same shape as the existing Quirks/Shadow runtime-only fields, not Attr), a JS accessor that reads/writes it directly (not via setAttr/removeAttr), and a new :indeterminate CSS pseudo-class wired into the same four sites :disabled already uses (Matches, the parser switch, the specificity count, the bare-compound-is-non-empty check).
• Found by sweeping a genuinely fresh corpus page (github.com's own homepage bundles — behaviors.js/element-registry.js/environment.js) after the existing ten-page corpus stopped turning up new gaps. Real usage: github.com's own behaviors.js sets checkbox.indeterminate = true on page load for its "select all" bulk-action tri-state checkboxes.
• Deliberately left out of scope, disclosed honestly: this engine's own checkbox painter (paint.paintCheckboxLike) already approximates checked as a plain accent-coloured fill with no checkmark glyph — a visually distinct indeterminate mark would mean inventing new iconography beyond what checked itself has. This round only fixes script/author-CSS correctness, not this engine's own default checkbox appearance.

Test plan

• Two new tests, git-stash-confirmed as a go vet COMPILE failure (the strongest class): css/selector_test.go's TestIndeterminatePseudo (mirrors TestCheckedPseudo/TestDisabledEnabledPseudo) and js/js_test.go's TestIndeterminateAccessor (confirms setting it does NOT touch getAttribute('indeterminate'), unlike every other boolean accessor on this element).
• go build ./... && go vet ./... && go test ./... clean.
• Coverage floors held (css 99.5%, layout 100%, paint 100%, dom 98.4%, paginate 100%).
• Bench vs. real headless Chrome: exactly flat/within already-documented noise across all ten pages, including github.com/golang/go itself (0.638, unchanged) — expected, pure JS/CSS-correctness change with no paint-visible effect by design.

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