Constant-pace travel: the playhead moves at a fixed speed, not fixed bars/leg

Paul: 'the playhead should always move at a constant pace; distance between nodes shouldn't matter.' The old model gave every leg exactly `pace` bars, so the traveler raced across long legs and crawled short ones.

▶ Play it: Mercury Interrogation — where the harmony organ has the least anywhere to hide https://stellate.app/?genre=bebop&seed=5266&path=694.13834,713.13867,675.13867&xdur=13.122

2a769ff https://github.com/aboard-io/stellate/commit/2a769ff27b178ed4f46efd3a37f692e652d6115d