[radio-button] Render content labels as a real label part (#376)

[MawiraIke]

Closes #376.

What

Since the 2.29.x part migration, radio-button rendered a string :label as a bare text node: no label element, no rc-radio-button-label, the theme's padding-left and the caller's :label-style/:label-class silently ignored — and (unreported until now) no on-click, so clicking the label stopped toggling the radio.

Root cause

Not the :impl "empty" the issue originally blamed — it's part/part's string shortcut: a string part-value is returned verbatim, bypassing the impl, the theme, and post-props. Because :top-level-arg? true conflates the :label content argument with the :parts override mechanism, :label "text" (the universal case) takes the wholesale-replacement path. (Correction posted on the issue: the "component :style lands on the input" observation was pre-existing parity, not a regression — pre-migration also merged class/style/attr onto the input.)

The fix

• New part/content? predicate (string | number | hiccup).
• radio-button wraps content labels in the themed ::rb/label span with the content as :children — the theme (rc-radio-button-label via the automatic part class, padding-left, :label-style/:label-class) and the click handler all apply again. Map/fn part values keep their override semantics.
• The label on-click rides :attr (where part/default actually emits handlers), restoring label-click toggling; the part structure declares the label's real :tag :span instead of :impl "empty".
• The bootstrap theme's explicit "rc-radio-button-label" literal is dropped — theme/re-com-meta already adds it automatically, and keeping both produced a duplicated class once the label element actually rendered.

Testing

• New test/re_com/radio_button_test.cljs: renders the component fn and walks the hiccup for the label part's props — asserts the element exists, the classes/styles/children/on-click all land, hiccup labels wrap, absent labels emit no part. RED on master (no label part in the tree), GREEN with the fix. Suite: 69 tests / 257 assertions, 0 failures.
• Verified in the demo app (DOM shows the restored span with padding and working :label-style) and downstream in app usage against this branch via a :local/root override changes render with correct spacing and clickable labels.
• Downstream note for app upgrades: apps that added wrapper-gap CSS workarounds for this bug (e.g. .rc-radio-button-wrapper {gap: 8px}) should remove them when bumping, or spacing doubles.

Sibling issue filed: #377 — checkbox has the same latent string-label bypass (invisible because its theme spaces via wrapper gap), fixable with the same part/content? pattern.

🤖 Generated with Claude Code