Waves is still in progress

Use oscillation as the entry point, lock down wave speed and wavelength, carry that into longitudinal sound and pitch-versus-loudness cues, add beats as the nearby-frequency superposition bridge, then move into Doppler shifts, interference, standing-wave patterns, and open-vs-closed air-column resonance without losing the live connection between motion and graph.

This completion page stays public, but the completion reflection only unlocks after every concept and authored checkpoint in this track is clear on this browser. The route still reads the same local concept records and solved challenge ids, and signed-in sync keeps reusing those records instead of inventing a separate track store.