Expose entered state names and animation durations

[jslauthor]

Two read-only accessors a host needs to sequence and measure several timelines itself, both already present in the runtime and simply not surfaced.
RiveStateMachine::stateChanged now carries the entered state's TIMELINE name instead of an empty string. A LayerState has no name of its own — StateMachineLayerComponent extends Core directly and the format has no name property for it — but an AnimationState knows the LinearAnimation it plays, and that name is what the editor shows on the state. States with no single timeline (entry / exit / any / blend) stay unnamed rather than inventing a label callers could come to depend on. The header's contract is updated, including a note that the first argument has always been the change index rather than a layer name: rive reports changes as a flat list across layers with no way to ask which layer each came from. RiveArtboard::animationDuration(name) / RiveView::animationDuration(name) report a clip's length in seconds WITHOUT instantiating it for playback. That's the point: a host driving a chain of timelines can weight them by real length before any of them has played, rather than discovering each duration only as it starts and lurching between equal-sized guesses. Misses and empty names return 0.
Tests: every enumerated animation reports a positive duration and misses return 0; the state-machine spy asserts at least one emission is named and that every name matches an enumerated animation, which is the regression guard for the empty-string behaviour.
Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com

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