feat: add FSI (Ehlers Fourier Series Indicator, TASC June 2019)

docs/indicators.md

@@ -441,6 +441,7 @@ Periodic pattern detection and dominant frequency extraction. Markets exhibit cy
 | [**CCYC**](../lib/cycles/ccyc/Ccyc.md) | Ehlers Cyber Cycle | 4-tap FIR + 2-pole high-pass IIR cycle extraction |
 | [**CG**](../lib/cycles/cg/Cg.md) | Ehlers Center of Gravity | Ehlers cycle measurement |
 | [**DSP**](../lib/cycles/dsp/Dsp.md) | Ehlers Detrended Synthetic Price | Cycle-isolated price component |
+| [**FSI**](../lib/cycles/fsi/Fsi.md) | Ehlers Fourier Series Indicator | 3-harmonic bandpass + amplitude-weighted reconstruction |
 | [**ACP**](../lib/cycles/acp/Acp.md) | Ehlers Autocorrelation Periodogram | Ehlers dominant cycle detection |
 | [**EBSW**](../lib/cycles/ebsw/Ebsw.md) | Ehlers Even Better Sinewave | Ehlers improved cycle indicator |
 | [**HOMOD**](../lib/cycles/homod/Homod.md) | Ehlers Homodyne Discriminator | Dominant cycle period tracking |

docs/pinescript.md

@@ -429,6 +429,7 @@ Markets oscillate. These indicators try to measure the oscillation itself — th
 | CCYC | Ehlers Cyber Cycle | [ccyc.pine](../lib/cycles/ccyc/ccyc.pine) |
 | CG | Ehlers Center of Gravity | [cg.pine](../lib/cycles/cg/cg.pine) |
 | DSP | Ehlers Detrended Synthetic Price | [dsp.pine](../lib/cycles/dsp/dsp.pine) |
+| FSI | Ehlers Fourier Series Indicator | [fsi.pine](../lib/cycles/fsi/fsi.pine) |
 | ACP | Ehlers Autocorrelation Periodogram | [acp.pine](../lib/cycles/acp/acp.pine) |
 | EBSW | Ehlers Even Better Sinewave | [ebsw.pine](../lib/cycles/ebsw/ebsw.pine) |
 | HOMOD | Ehlers Homodyne Discriminator | [homod.pine](../lib/cycles/homod/homod.pine) |