The full pure-C facade used by the app: node/project/timeline/viewer/
undo/task/events/serializer/playback/preview/renderer/gizmo/color/
audio/footage/proxy/encoding/exporter/config/disk/ipc/plugin/worker
families, plus undo-group semantics, display renderer handles,
NodeFactory accessors, and per-family pure-C engine tests.
- clip-level loop mode (off/loop/clamp), undoable
- per-channel audio levels as one-frame RMS at a timestamp
- waveform min/max buckets over a footage range, rendered on demand
- two real fixes uncovered by this family: conform completion signals
were starved by msleep-only waits (audio renders always came back
empty), and incomplete tickets from conform-pending renders are now
retried until the conform is ready