The two genuinely-missing arithmetic primitives (Mul = two-stream f64 product,
Sqrt = one-input f64 root, negatives clamped to 0). They are the building blocks
for the volatility stop as a composition (rolling EWMA stddev), replacing the
fused VolStop node. Also corrects plan 0066 Task 3 (the VolStop removal must
migrate its stage1_r_e2e.rs caller — a false premise the implementer caught).
refs #117#119
