tests: coverage round 7 (audio, distort, filter/keying, math/transition nodes)
- node_audio_test: PanNode/VolumeNode sample math, TimeInput, ValueNode
- node_distort_test: transform/crop/flip/cornerpin/mask/ripple/swirl/
tile/wave inputs, shader code, value jobs, resolution autoscale math
- node_filter_keying_test: opacity/blur/dropshadow/mosaic/stroke jobs,
chroma/color-difference key and despill value composition
- node_math_transition_test: TrigonometryNode ops, MergeNode,
TransitionBlock offsets/progress/crossfade math, SubtitleBlock,
traverser globals and table caching