Type Alias HrZoneEntry

HrZoneEntry:
| { at: string; maxHr: number }
| { at: string; bounds: [number, number, number, number] }

HR-zone basis: either a max HR (zones derived from %s) or explicit bounds (the four Z1/Z2, Z2/Z3, Z3/Z4, Z4/Z5 boundaries in bpm).