Resonant Drum Field (case study)

This product includes software adapted from Countercurrent
  https://github.com/AdemVessell/countercurrent  (commit 265e9544d345d64c99f0788b847a3d890c30fc6a)
  licensed under the Apache License, Version 2.0 (see LICENSE).

Adapted files (modified; see EVIDENCE.md for diffs):
  src/physics/reference.js   -> src/physics/multi.js
  tests/reference.test.mjs   -> tests/multi.test.mjs
Verbatim upstream copies kept in upstream-originals/ for provenance.

The upstream repository ships no NOTICE file; its THIRD_PARTY_NOTICES.md is kept
verbatim at upstream-originals/THIRD_PARTY_NOTICES.md. Three.js (MIT) is a runtime dependency.