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.

crates/wickra-core/src/indicators/apo.rs

@@ -0,0 +1,183 @@
+//! Absolute Price Oscillator (APO).
+
+use crate::error::{Error, Result};
+use crate::indicators::ema::Ema;
+use crate::traits::Indicator;
+
+/// Absolute Price Oscillator — the raw difference between a fast and a slow
+/// `EMA`. This is MACD's line without the signal-EMA — useful when only the
+/// momentum-direction reading is needed.
+///
+/// ```text
+/// APO_t = EMA(close, fast)_t − EMA(close, slow)_t
+/// ```
+///
+/// Default parameters mirror MACD: `(fast = 12, slow = 26)`. `fast` must be
+/// strictly less than `slow`.
+///
+/// # Example
+///
+/// ```
+/// use wickra_core::{Apo, Indicator};
+///
+/// let mut apo = Apo::new(12, 26).unwrap();
+/// let mut last = None;
+/// for i in 0..80 {
+///     last = apo.update(100.0 + f64::from(i));
+/// }
+/// assert!(last.is_some());
+/// ```
+#[derive(Debug, Clone)]
+pub struct Apo {
+    fast_period: usize,
+    slow_period: usize,
+    fast: Ema,
+    slow: Ema,
+}
+
+impl Apo {
+    /// # Errors
+    /// - [`Error::PeriodZero`] if either period is zero.
+    /// - [`Error::InvalidPeriod`] if `fast >= slow`.
+    pub fn new(fast: usize, slow: usize) -> Result<Self> {
+        if fast == 0 || slow == 0 {
+            return Err(Error::PeriodZero);
+        }
+        if fast >= slow {
+            return Err(Error::InvalidPeriod {
+                message: "APO fast period must be strictly less than slow",
+            });
+        }
+        Ok(Self {
+            fast_period: fast,
+            slow_period: slow,
+            fast: Ema::new(fast)?,
+            slow: Ema::new(slow)?,
+        })
+    }
+
+    /// MACD-style defaults: `(fast = 12, slow = 26)`.
+    pub fn classic() -> Self {
+        Self::new(12, 26).expect("classic APO parameters are valid")
+    }
+
+    /// Configured `(fast, slow)`.
+    pub const fn periods(&self) -> (usize, usize) {
+        (self.fast_period, self.slow_period)
+    }
+}
+
+impl Indicator for Apo {
+    type Input = f64;
+    type Output = f64;
+
+    fn update(&mut self, input: f64) -> Option<f64> {
+        // Feed both EMAs on every input so the slow one warms in parallel.
+        let f = self.fast.update(input);
+        let s = self.slow.update(input);
+        Some(f? - s?)
+    }
+
+    fn reset(&mut self) {
+        self.fast.reset();
+        self.slow.reset();
+    }
+
+    fn warmup_period(&self) -> usize {
+        // Slow EMA dominates; both EMAs emit at their `period` th input.
+        self.slow_period
+    }
+
+    fn is_ready(&self) -> bool {
+        self.slow.is_ready()
+    }
+
+    fn name(&self) -> &'static str {
+        "APO"
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::traits::BatchExt;
+    use approx::assert_relative_eq;
+
+    #[test]
+    fn rejects_zero_period() {
+        assert!(matches!(Apo::new(0, 26), Err(Error::PeriodZero)));
+        assert!(matches!(Apo::new(12, 0), Err(Error::PeriodZero)));
+    }
+
+    #[test]
+    fn rejects_fast_geq_slow() {
+        assert!(matches!(Apo::new(26, 12), Err(Error::InvalidPeriod { .. })));
+        assert!(matches!(Apo::new(12, 12), Err(Error::InvalidPeriod { .. })));
+    }
+
+    #[test]
+    fn accessors_and_metadata() {
+        let apo = Apo::classic();
+        assert_eq!(apo.periods(), (12, 26));
+        assert_eq!(apo.warmup_period(), 26);
+        assert_eq!(apo.name(), "APO");
+    }
+
+    #[test]
+    fn classic_factory() {
+        assert_eq!(Apo::classic().periods(), (12, 26));
+    }
+
+    #[test]
+    fn constant_series_converges_to_zero() {
+        // Both EMAs reproduce the constant exactly, so APO is 0.
+        let mut apo = Apo::new(3, 5).unwrap();
+        let out = apo.batch(&[42.0_f64; 30]);
+        for v in out.iter().skip(4).flatten() {
+            assert_relative_eq!(*v, 0.0, epsilon = 1e-12);
+        }
+    }
+
+    #[test]
+    fn warmup_emits_first_value_at_slow_period() {
+        let mut apo = Apo::new(2, 4).unwrap();
+        assert_eq!(apo.warmup_period(), 4);
+        for i in 1..=3 {
+            assert_eq!(apo.update(f64::from(i)), None);
+        }
+        assert!(apo.update(4.0).is_some());
+    }
+
+    #[test]
+    fn pure_uptrend_is_positive() {
+        // Fast EMA leads the slow EMA on an uptrend, so APO > 0.
+        let mut apo = Apo::classic();
+        let prices: Vec<f64> = (1..=200).map(f64::from).collect();
+        let out = apo.batch(&prices);
+        let last = out.iter().rev().flatten().next().unwrap();
+        assert!(*last > 0.0, "APO on uptrend should be positive: {last}");
+    }
+
+    #[test]
+    fn batch_equals_streaming() {
+        let prices: Vec<f64> = (1..=120)
+            .map(|i| 100.0 + (f64::from(i) * 0.2).sin() * 5.0)
+            .collect();
+        let mut a = Apo::classic();
+        let mut b = Apo::classic();
+        assert_eq!(
+            a.batch(&prices),
+            prices.iter().map(|p| b.update(*p)).collect::<Vec<_>>()
+        );
+    }
+
+    #[test]
+    fn reset_clears_state() {
+        let mut apo = Apo::classic();
+        apo.batch(&(1..=80).map(f64::from).collect::<Vec<_>>());
+        assert!(apo.is_ready());
+        apo.reset();
+        assert!(!apo.is_ready());
+        assert_eq!(apo.update(1.0), None);
+    }
+}

crates/wickra-core/src/indicators/mod.rs

@@ -7,6 +7,7 @@
 mod accelerator_oscillator;
 mod adl;
 mod adx;
+mod apo;
 mod aroon;
 mod aroon_oscillator;
 mod atr;
@@ -79,6 +80,7 @@ mod zlema;
 pub use accelerator_oscillator::AcceleratorOscillator;
 pub use adl::Adl;
 pub use adx::{Adx, AdxOutput};
+pub use apo::Apo;
 pub use aroon::{Aroon, AroonOutput};
 pub use aroon_oscillator::AroonOscillator;
 pub use atr::Atr;

crates/wickra-core/src/lib.rs

@@ -44,7 +44,7 @@ pub mod indicators;
 
 pub use error::{Error, Result};
 pub use indicators::{
-    AcceleratorOscillator, Adl, Adx, AdxOutput, Aroon, AroonOscillator, AroonOutput, Atr,
+    AcceleratorOscillator, Adl, Adx, AdxOutput, Apo, Aroon, AroonOscillator, AroonOutput, Atr,
     AtrTrailingStop, AwesomeOscillator, BalanceOfPower, BollingerBands, BollingerBandwidth,
     BollingerOutput, Cci, ChaikinMoneyFlow, ChaikinOscillator, ChaikinVolatility, ChandeKrollStop,
     ChandeKrollStopOutput, ChandelierExit, ChandelierExitOutput, ChoppinessIndex, Cmo, Coppock,