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.

README.md

@@ -109,7 +109,7 @@ python -m benchmarks.compare_libraries
 
 ## Indicators
 
-71 streaming-first indicators across eight families. Every one passes the
+72 streaming-first indicators across eight families. Every one passes the
 `batch == streaming` equivalence test, reference-value tests, and reset
 semantics tests.
 
@@ -118,7 +118,7 @@ semantics tests.
 | Moving Averages      | SMA, EMA, WMA, DEMA, TEMA, HMA, KAMA, SMMA, TRIMA, ZLEMA, T3, VWMA |
 | Momentum Oscillators | RSI (Wilder), Stochastic, CCI, ROC, Williams %R, MFI, Awesome Oscillator, MOM, CMO, TSI, PMO, StochRSI, Ultimate Oscillator |
 | Trend & Directional  | MACD, ADX (+DI/-DI), Aroon, TRIX, Aroon Oscillator, Vortex, Mass Index, Choppiness Index, Vertical Horizontal Filter |
-| Price Oscillators    | PPO, DPO, Coppock, Accelerator Oscillator, Balance of Power |
+| Price Oscillators    | PPO, DPO, Coppock, Accelerator Oscillator, Balance of Power, APO |
 | Volatility & Bands   | ATR, Bollinger Bands, Keltner Channels, Donchian Channels, NATR, StdDev, Ulcer Index, Historical Volatility, Bollinger Bandwidth, %B, True Range, Chaikin Volatility |
 | Trailing Stops       | Parabolic SAR, SuperTrend, Chandelier Exit, Chande Kroll Stop, ATR Trailing Stop |
 | Volume               | OBV, VWAP (cumulative + rolling), ADL, Volume-Price Trend, Chaikin Money Flow, Chaikin Oscillator, Force Index, Ease of Movement |