Function fbm2D

fbm2D(
    x: number,
    y: number,
    octaves?: number,
    lacunarity?: number,
    gain?: number,
): number
Multi-octave fractal Brownian motion (fBm) using 2D value noise.

Combines several octaves of noise at progressively higher frequencies and lower amplitudes to produce a natural-looking signal.
Parameters
- x: number
  X coordinate.
- y: number
  Y coordinate.
- octaves: number = 4
  Number of noise octaves (default 4). More = finer detail.
- lacunarity: number = 2
  Frequency multiplier per octave (default 2). Controls how quickly frequency increases.
- gain: number = 0.5
  Amplitude multiplier per octave (default 0.5). Controls how quickly higher-frequency octaves fade out.
Returns number
Composite noise value in approximately [0, 1].
Example
```
// Slowly evolving value suitable for animating line opacity over time
const opacity = fbm2D(beat * 0.1, lineIndex * 0.3) * 255;
```
- Defined in math.ts:504