feat(component): always emit variant suffix in uiButton class

[bpamiri]

Summary

• uiButton now always emits the variant suffix in its compound class, including the default primary variant
• Bare uiButton(text=\"Save\") → <button class=\"btn-primary\"> (was class=\"btn\")
• Sized primary follows the rule: size=\"sm\" → class=\"btn-sm-primary\" (was class=\"btn-sm\"); same for lg and icon-only

Why

Rendered HTML now self-documents the variant. Inspecting class=\"btn\" in DevTools could previously mean either "explicit primary" or "author forgot to set the variant" — those should be visually distinguishable. basecoat-css ships matching selectors for every size×variant combination (.btn-primary, .btn-sm-primary, .btn-lg-primary, .btn-icon-primary), so visual rendering is identical to bare btn.

Source

Discovered during a Wheels Tutorial fresh-VM bake. The chapter-8 "Bonus: wheels-basecoat" checkpoint asserted class=\"btn btn-primary\" against rendered output; the silent-skip behavior made the assertion impossible to satisfy. The companion chapter doc fix lands separately in wheels-dev/wheels.

Wheels Tutorial Finding #5.

Test plan

• Tag 1.0.5 and verify the wheels-packages registry picks it up
• On a fresh wheels packages add wheels-basecoat, confirm chapter 8 checkpoint fix(component): $uiBuildId / $uiLucideIcon must be public for PackageLoader #3 (`grep -oE '<button[^>]class="btn[^\"]"[^>]*>Post comment'`) returns a match including btn-primary
• Confirm visual rendering is unchanged in a real browser (basecoat.css .btn-primary rule == .btn rule)
• Updated unit tests pass (`renders primary button with btn-primary class`, `renders small size with primary variant`, `renders icon-only primary button`)

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