feat(ui): animate segmented control with solid-motion and fix button hit-testing

[kitlangton]

Why

SegmentedControl switched active segments statically without an indicator transition, did not brighten inactive segment labels on hover, and allowed child label nodes to intercept pointer hit-testing and text selection. Additionally, button.css, icon-button.css, and tabs-current.css defined an unscoped universal * rule that leaked font-smoothing and text-rendering: geometricPrecision across the entire document, and Button child elements could split hit-testing from the <button>.

What Changes

• Animated active pill (SegmentedControl): Renders a <motion.span data-slot="segmented-control-v2-indicator" aria-hidden="true" /> driven by @kitlangton/solid-motion ({ type: "spring", visualDuration: 0.15, bounce: 0 } by default, configurable via transition), jumping immediately on initial mount, resize, or prefers-reduced-motion and spring-animating between active segments.
• Pre-measurement fallback: Retains :not([data-ready]) active-item background and border-shadow styling before layout measurement completes.
• Segmented control interaction & hit-testing: Brightens inactive enabled segment labels on hover (:hover:not(:disabled):not([data-pressed])), disables text selection on the control and items, and sets pointer-events: none on [data-slot="segmented-control-v2-item-label"].
• Button hit-testing & scoped font smoothing: Adds [data-component="button-v2"] > * { pointer-events: none; } and scopes -webkit-font-smoothing: antialiased and -moz-osx-font-smoothing: grayscale to [data-component="button-v2"], [data-component="icon-button-v2"], and [data-component="tabs-v2"] instead of universal *.

Scope

Covers @opencode/ui SegmentedControl (SegmentedControlV2 / SegmentedControlItemV2 aliases included), button.css, icon-button.css, and tabs-current.css.

Verification

bun run --cwd packages/ui typecheck
bun run --cwd packages/ui test
bun run --cwd packages/ui build
bun run lint:changed

• packages/ui typecheck (tsgo --noEmit), build (tsc -p tsconfig.build.json), and unit tests (135 passing across 8 files, including segmented-control.test.ts) pass.
• bun run check (repo-wide oxlint, ast-grep, sdk-docs, and turbo typecheck across 36 packages) passed on push.