Revert the fixed-window approach from v0.2.3 — EMA now collects
`length` valid (non-NaN) values starting from the first valid bar.
This fixes chained EMAs on derived series (MACD signal, TRIX,
DEMA/TEMA, Elder Force) where leading NaN caused complete
initialization failure or wrong seed values.

Closes #81

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>