css+paint: honour appearance:none on checkbox/radio controls

[tannevaled]

Summary

• paintCheckboxLike had no *css.Style parameter at all, so a checkbox/radio with appearance:none (the standard custom-toggle-switch pattern) always painted this engine's generic hardcoded square instead of the author's own cascaded style.
• Confirmed real, load-bearing trigger: developer.mozilla.org's own <mdn-switch> toggle-switch component sets appearance:none;background-color:...;background-image:radial-gradient(...);border-radius:9999px unconditionally on a real <input type=checkbox> (independently fetched and verified against the live page's own declarative-shadow-DOM markup, not trusted from a delegated audit alone).
• Adds a new non-inherited css.Style.AppearanceNone field (backing appearance/-webkit-appearance: none; other keywords have no confirmed trigger and are left unsupported). When set, paintCheckboxLike now paints like any other styled box — background colour, background-image layers (gradient/bitmap, reusing paintBackgroundLayers generalized off *layout.Box), border-radius, and per-side border — instead of the generic square. Without appearance:none, nothing changes.
• Corrected a stale css/ua.go comment that predicted paint would never consult a checkbox's Background/Border fields — no longer true for the appearance:none branch.

See the new FIDELITY.md entry (round 146, "Known gaps" also updated) for full detail, including the live-page verification (an isolated repro of the real <mdn-switch> markup, rendered standalone) and the full per-page bench attribution (4/10 pages moved, each individually confirmed to be pre-existing, unrelated live-content churn — none reach a checkbox at all).

Test plan

• go build ./... / go vet ./... clean
• go test ./... green, all five coverage floors held (css 99.5%, paint/layout/paginate 100.0%, dom 98.4%)
• Git-stash-confirmed: reverting the four production files alone fails to even compile in both css and paint
• 8 new tests: TestApplyAppearanceProperty (css) + 6 new paint/formcontrol_test.go cases (regression guard without appearance:none, background colour, background-image gradient, border + bare/no-style, border-radius)
• Visually verified on an isolated repro of MDN's own real <mdn-switch> markup: unchecked renders a grey pill with a white knob on the left, checked a blue pill with the knob slid right (both the paint fix and the :checked colour/position swap, which needs no special-casing)
• Plain unstyled checkboxes re-render identically to before this change
• Full 10-page bench run against headless Chrome; every moved page individually attributed to unrelated, already-documented noise

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