Brummel
ae1d4564f5
Add Ema, the std-lib's first recursive/stateful producer: exponential moving average with alpha = 2/(length+1). The building block MACD (and DEMA, MACD-histogram, etc.) needs, and a worked example of a node whose param sizes its math, not its input window. Seeding is ratified as SMA-seeded (ta-lib convention): the EMA stays silent until it has seen length samples, seeds with their SMA, then runs the recurrence. Chosen over a first-value seed for three substantive reasons -- warm-up consistency with Sma (both emit None until length samples), parity with the EMA traders expect (ta-lib MACD seeds this way), and statistical honesty (no average claimed from a single observation). A first-value seed was the proof-of-concept convenience; rejected. Performance: recursive state means lookback = 1 (length sizes alpha, not the window); eval is O(1) time and state via ema += alpha*(x - ema), with no hot-path allocation (the output buffer is reused). Tested: warm-up-then-smooth (a step input separates it from a plain SMA), length-1 identity, label, and factory/schema agreement.