tsi: seed momentum[0] with NaN instead of 0 to match PineScript
(close - close[1] is na on bar 0).

ema: use fixed-width window of first `length` bars for SMA seed,
averaging only non-NaN values. Previously skipped NaN positions
entirely, shifting the seed index by one per NaN.

Closes #79
Closes #80

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