Synchronization between Program and Renderer

Currently, a `Program` gets the time elapsed since the last tick as a `Duration` argument to `tick`.

A Synthesizer gets the total number of samples played with every call to `synthesize`.

There should be some kind of guarantee that these things line up, and they should be directly relatable.

Start by locating Program::tick and Synthesizer::synthesize, then trace how elapsed time and total samples are produced and consumed. Define what synchronization guarantee and direct relationship are required; the issue is done when both interfaces provide a documented, testable correspondence.

Written by the indexing model from the issue text.