Recursive Filters: SMA, EMA, Low‑Pass, and a Tiny Kalman

Too basic, I'm not sure anyone needs that to solve the task as basic would need to read an article, it's obvious.

> Tune by eyeballing lag vs. noise.

In any serious installations (more than a homelab) you want some numeric metrics, not eyeballing. E.g. management or next engineer may reasonably ask "why alpha is 0.8? what's the rationale, why not 0.85", you want some formula to show it's what we want.