fix: correct SFX ma_std_dev warmup issue by filtering None values (closes #154)

[botbikamordehai2-sketch]

What

The SFX indicator was producing an anomalous first value for ma_std_dev (e.g., 9992.06 instead of ~2.08). This happened because ma_std_dev was constructed with input_indicator=self.std_dev, causing it to receive None values during std_dev's warmup period. These None values corrupted the internal state of the moving average.

Fix

Removed input_indicator=self.std_dev from the ma_std_dev constructor and instead registered it as a managed sequence via add_managed_sequence. The _calculate_new_value method already manually reads from ma_std_dev, so we need to feed it valid values only. The managed sequence approach ensures ma_std_dev only receives valid (non-None) values from std_dev.

Closes #154