Function beatsToSeconds

beatsToSeconds(beat: number, map: TempoMap): number
Wall-clock seconds from the start of a tempo map to a beat.

The result integrates every tempo segment the span crosses; it is not the tempo in effect at beat applied to the whole span.

A beat before the map's first event reports negative elapsed time, at the opening tempo: a pickup sounds before the downbeat, and asking when it sounds is a fair question.
Parameters
- beat: number
  Position in quarter-note beats.
- map: TempoMap
  The tempo map; its first event is the time origin.
Returns number
Elapsed seconds, 0 at the map's first event and negative before it.
Throws
If the map is empty, unsorted, or carries a tempo outside 0.1..1000 quarter-note beats per minute.
Example
```
import { beatsToSeconds } from '@libraz/libcantus';
const map = [{ startBeat: 0, bpm: 120 }, { startBeat: 4, bpm: 60 }];
beatsToSeconds(8, map); // 6 = 4 beats at 120 + 4 beats at 60
```
- Defined in src/core/tempo/index.ts:203