feat(apo): add Absolute Price Oscillator

EMA(close, fast) - EMA(close, slow). Like MACD without the signal EMA.
Defaults to (fast = 12, slow = 26); fast must be strictly less than
slow.

Touchpoints: apo.rs + mod.rs + lib.rs re-export, PyApo + __init__.py
+ test_new_indicators SCALAR + test_known_values flat reference,
ApoNode + index.d.ts/index.js + indicators.test.js factory + reference,
WasmApo via scalar macro, scalar-fuzz target, README + CHANGELOG.

bindings/python/tests/test_new_indicators.py

@@ -51,6 +51,7 @@ SCALAR = [
     (ta.PMO, (35, 20)),
     (ta.StochRSI, (14, 14)),
     (ta.PPO, (12, 26)),
+    (ta.APO, (12, 26)),
     (ta.DPO, (20,)),
     (ta.Coppock, (14, 11, 10)),
     (ta.StdDev, (20,)),