Expose state machine graph, entered state names, and timeline introspection

[jslauthor]

Three read-only surfaces a host needs to let a .riv's state machine own its sequencing while the host still measures and reacts to it — all already present in the runtime and simply not surfaced.

RiveArtboard::stateMachineGraph(name) (+ RiveView passthrough) returns a machine's static structure WITHOUT instantiating it: entry timeline, every AnimationState's timeline name, and transitions resolved from LayerState* to {from, to} name pairs (entry/exit/any/blend report "", matching the stateChanged contract). A host can now derive a clip sequence from the graph itself instead of duplicating it in code — so adding or reordering states in the .riv needs no host change.

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. This is also the host's transition-interception point: every handoff is observable as a from -> to pair.

RiveStateMachine::currentStateTime / currentStateTimeline (read-only Q_PROPERTYs, published per advance) expose the live playhead within the state's active clip via currentAnimationByIndex(). Under a state machine RiveView's currentFrame is 0 by design — a graph has no single scrubbable timeline — but a host progress bar still needs to measure playback, and these two give it position-within-state to combine with the graph-derived ordering and per-clip durations.

RiveArtboard::animationDuration(name) / RiveView::animationDuration(name) report a clip's length in seconds WITHOUT instantiating it for playback; animationLoops(name) reports whether it repeats (loop or ping-pong), which is how a host tells a graph spine's loop tail from a one-shot end. Misses and empty names return 0 / false.

Tests: graph shape + endpoints cross-checked against animationNames with miss/mint-after-read contracts; playhead populated after first advance and bounded by the clip's real duration; loop flag cross-checked against each clip's own setting; the state-machine spy asserts at least one emission is named and every name matches an enumerated animation (the regression guard for the empty-string behaviour); render smoke covers the RiveView no-source contract. Full suite: 38/38.

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