ComposableView actions: callbacks back into Swift

[colemancda]

Completes the custom-view mechanism (#39) with the "coordinator" half — a custom composable can now send events back into Swift state, so interop is two-way.

API

ComposableView gains an actions: parameter — a dictionary of named ComposableActions, each a typed closure matching the fixed bridge surface:

ComposableView("RatingBar",
    props: ["rating": .double(stars), "max": 5],
    actions: ["onRatingChanged": .double { stars = $0 }])

Each action registers a callback in the resolve context (exactly like Button's tap) and crosses as an id prop. On the Kotlin side, Props exposes typed action lambdas — props.doubleAction("onRatingChanged") returns a (Double) -> Unit that dispatches back to Swift; the factory never sees the id or the bridge:

val onChanged = props.doubleAction("onRatingChanged")
RatingBar(context).setOnRatingBarChangeListener { _, value, fromUser ->
    if (fromUser) onChanged?.invoke(value.toDouble())
}

Verification

• swift test — new test invoking a registered action through the callback registry, 61 total passing
• Emulator (Custom Views screen): the demo RatingBar is now interactive — dragging the stars updates Swift @State (drag → 1.5, and the sibling Text reads "1.5 / 5"), while the ±½ buttons drive it the other way. Both directions share one state with no feedback loop (the fromUser guard filters programmatic updates).

Deferred

Actions are per-event fire-and-forget (no return values back to the composable); coordinator-style object state lives in the Kotlin factory's remember.