feat(family-12): add 13 Statistik/Regression indicators (#51)

* feat(family-12): add 13 Statistik/Regression indicators

Brings the Price Statistics family to 20 indicators (7 → 20) and the
total catalogue to 84 (71 → 84). Every indicator ships in the Rust
core plus Python, Node, and WASM bindings with full streaming ↔ batch
parity, fuzz coverage, and benches.

Scalar (f64 → f64):
- Variance, CoefficientOfVariation: rolling population variance and
  its dimensionless ratio with the mean. O(1) updates.
- Skewness, Kurtosis: rolling Pearson skewness and excess kurtosis,
  derived from running sums of x, x², x³, x⁴ via the binomial
  identities — also O(1) per bar.
- StandardError, DetrendedStdDev: standard error of estimate (n − 2)
  and population StdDev (n) of OLS residuals, sharing the LinReg
  O(1) sliding sums.
- RSquared: coefficient of determination of the rolling OLS fit; the
  trend-quality filter, clamped to [0, 1].
- MedianAbsoluteDeviation: robust dispersion estimator; O(period log
  period) per emission via two in-place sorts of a reusable scratch
  buffer.
- Autocorrelation(period, lag): rolling lag-k Pearson autocorrelation.
- HurstExponent(period, chunks): R/S-analysis trend-persistence
  estimator clamped to [0, 1].

Pair indicators (Input = (f64, f64)):
- PearsonCorrelation: rolling cross-series Pearson, O(1).
- Beta: rolling OLS slope of asset vs. benchmark (CAPM).
- SpearmanCorrelation: rolling rank correlation with mid-rank tie
  handling; O(period log period).

Touchpoints:
- crates/wickra-core: 13 new indicator modules + mod.rs / lib.rs
  re-exports.
- bindings/python: pyclasses + add_class registration + __init__.py
  import & __all__ updates. The pair indicators expose
  update(x, y) and batch(x, y) over two equally-sized numpy arrays.
- bindings/node: scalar indicators via node_scalar_indicator! macro;
  pair indicators via new node_pair_indicator! macro; explicit
  structs for Autocorrelation and HurstExponent (two-arg ctors).
  index.js extended with the new exports.
- bindings/wasm: scalar wrappers via wasm_scalar_indicator!; pair
  wrappers via new wasm_pair_indicator! macro.
- fuzz: every scalar drove through the generic helper; pair
  indicators stress-tested by pairing adjacent samples of the fuzz
  input.
- Python tests (test_new_indicators.py): added to SCALAR
  parametrisation, plus algebraic reference values
  (variance of [2,4,6] = 8/3, MAD ignoring outlier = 0, monotone
  non-linear Spearman = 1, two-to-one Beta = 2, etc.) and a
  streaming-vs-batch test for the pair indicators.
- Node tests (indicators.test.js): extended the scalar factories
  map and added a pair-indicator section with the same algebraic
  reference values.
- crates/wickra/benches: bench_scalar entries for all 10 single-
  input new indicators.
- README: counter 71 → 84; Price Statistics family-table row
  expanded with the 13 new indicators.
- CHANGELOG: Unreleased section documents the family addition.

Wiki drafts (ghost-ignored, manual sync to wickra.wiki at release
time): indicator-ideas/families/wiki/family-12-statistik-regression/
contains 13 deep-dive pages plus _Sidebar / Indicators-Overview /
Warmup-Periods / Home fragments for the curator merge.

cargo check --workspace --all-features: clean.

* fix(family-12): remove unreachable defensive guards in hurst_exponent

The three guards (m < 2 continue, end > buf.len() break, denom == 0.0
return) are by-construction unreachable given the constructor invariant
period >= 2 * chunks: m = period / k for k in 1..=chunks always
satisfies m >= 2 and end = (c+1) * m <= k * m <= period = buf.len(),
and m_1 = period and m_2 = period / 2 are always distinct so the slope
denominator is strictly positive. Removing them brings codecov/patch
back to 100%.

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

@@ -20,9 +20,11 @@ mod aroon_oscillator;
 mod atr;
 mod atr_bands;
 mod atr_trailing_stop;
+mod autocorrelation;
 mod awesome_oscillator;
 mod awesome_oscillator_histogram;
 mod balance_of_power;
+mod beta;
 mod bollinger;
 mod bollinger_bandwidth;
 mod camarilla_pivots;
@@ -37,6 +39,7 @@ mod choppiness_index;
 mod classic_pivots;
 mod cmf;
 mod cmo;
+mod coefficient_of_variation;
 mod connors_rsi;
 mod coppock;
 mod cybernetic_cycle;
@@ -45,6 +48,7 @@ mod decycler_oscillator;
 mod dema;
 mod demand_index;
 mod demark_pivots;
+mod detrended_std_dev;
 mod donchian;
 mod donchian_stop;
 mod double_bollinger;
@@ -68,6 +72,7 @@ mod hilo_activator;
 mod historical_volatility;
 mod hma;
 mod hurst_channel;
+mod hurst_exponent;
 mod ichimoku;
 mod inertia;
 mod instantaneous_trendline;
@@ -76,6 +81,7 @@ mod jma;
 mod kama;
 mod keltner;
 mod kst;
+mod kurtosis;
 mod kvo;
 mod laguerre_rsi;
 mod linreg;
@@ -88,6 +94,7 @@ mod mama;
 mod market_facilitation_index;
 mod mass_index;
 mod mcginley_dynamic;
+mod median_absolute_deviation;
 mod median_price;
 mod mfi;
 mod mom;
@@ -95,6 +102,7 @@ mod natr;
 mod nvi;
 mod obv;
 mod parkinson;
+mod pearson_correlation;
 mod percent_b;
 mod percentage_trailing_stop;
 mod pgo;
@@ -102,6 +110,7 @@ mod pmo;
 mod ppo;
 mod psar;
 mod pvi;
+mod r_squared;
 mod renko_trailing_stop;
 mod roc;
 mod rogers_satchell;
@@ -111,9 +120,12 @@ mod rvi;
 mod rvi_volatility;
 mod rwi;
 mod sine_wave;
+mod skewness;
 mod sma;
 mod smi;
 mod smma;
+mod spearman_correlation;
+mod standard_error;
 mod standard_error_bands;
 mod starc_bands;
 mod stc;
@@ -147,6 +159,7 @@ mod ttm_squeeze;
 mod typical_price;
 mod ulcer_index;
 mod ultimate_oscillator;
+mod variance;
 mod vertical_horizontal_filter;
 mod vidya;
 mod volty_stop;
@@ -186,9 +199,11 @@ pub use aroon_oscillator::AroonOscillator;
 pub use atr::Atr;
 pub use atr_bands::{AtrBands, AtrBandsOutput};
 pub use atr_trailing_stop::AtrTrailingStop;
+pub use autocorrelation::Autocorrelation;
 pub use awesome_oscillator::AwesomeOscillator;
 pub use awesome_oscillator_histogram::AwesomeOscillatorHistogram;
 pub use balance_of_power::BalanceOfPower;
+pub use beta::Beta;
 pub use bollinger::{BollingerBands, BollingerOutput};
 pub use bollinger_bandwidth::BollingerBandwidth;
 pub use camarilla_pivots::{Camarilla, CamarillaPivotsOutput};
@@ -203,6 +218,7 @@ pub use choppiness_index::ChoppinessIndex;
 pub use classic_pivots::{ClassicPivots, ClassicPivotsOutput};
 pub use cmf::ChaikinMoneyFlow;
 pub use cmo::Cmo;
+pub use coefficient_of_variation::CoefficientOfVariation;
 pub use connors_rsi::ConnorsRsi;
 pub use coppock::Coppock;
 pub use cybernetic_cycle::CyberneticCycle;
@@ -211,6 +227,7 @@ pub use decycler_oscillator::DecyclerOscillator;
 pub use dema::Dema;
 pub use demand_index::DemandIndex;
 pub use demark_pivots::{DemarkPivots, DemarkPivotsOutput};
+pub use detrended_std_dev::DetrendedStdDev;
 pub use donchian::{Donchian, DonchianOutput};
 pub use donchian_stop::{DonchianStop, DonchianStopOutput};
 pub use double_bollinger::{DoubleBollinger, DoubleBollingerOutput};
@@ -234,6 +251,7 @@ pub use hilo_activator::HiLoActivator;
 pub use historical_volatility::HistoricalVolatility;
 pub use hma::Hma;
 pub use hurst_channel::{HurstChannel, HurstChannelOutput};
+pub use hurst_exponent::HurstExponent;
 pub use ichimoku::{Ichimoku, IchimokuOutput};
 pub use inertia::Inertia;
 pub use instantaneous_trendline::InstantaneousTrendline;
@@ -242,6 +260,7 @@ pub use jma::Jma;
 pub use kama::Kama;
 pub use keltner::{Keltner, KeltnerOutput};
 pub use kst::{Kst, KstOutput};
+pub use kurtosis::Kurtosis;
 pub use kvo::Kvo;
 pub use laguerre_rsi::LaguerreRsi;
 pub use linreg::LinearRegression;
@@ -254,6 +273,7 @@ pub use mama::{Mama, MamaOutput};
 pub use market_facilitation_index::MarketFacilitationIndex;
 pub use mass_index::MassIndex;
 pub use mcginley_dynamic::McGinleyDynamic;
+pub use median_absolute_deviation::MedianAbsoluteDeviation;
 pub use median_price::MedianPrice;
 pub use mfi::Mfi;
 pub use mom::Mom;
@@ -261,6 +281,7 @@ pub use natr::Natr;
 pub use nvi::Nvi;
 pub use obv::Obv;
 pub use parkinson::ParkinsonVolatility;
+pub use pearson_correlation::PearsonCorrelation;
 pub use percent_b::PercentB;
 pub use percentage_trailing_stop::PercentageTrailingStop;
 pub use pgo::Pgo;
@@ -268,6 +289,7 @@ pub use pmo::Pmo;
 pub use ppo::Ppo;
 pub use psar::Psar;
 pub use pvi::Pvi;
+pub use r_squared::RSquared;
 pub use renko_trailing_stop::RenkoTrailingStop;
 pub use roc::Roc;
 pub use rogers_satchell::RogersSatchellVolatility;
@@ -277,9 +299,12 @@ pub use rvi::Rvi;
 pub use rvi_volatility::RviVolatility;
 pub use rwi::{Rwi, RwiOutput};
 pub use sine_wave::SineWave;
+pub use skewness::Skewness;
 pub use sma::Sma;
 pub use smi::Smi;
 pub use smma::Smma;
+pub use spearman_correlation::SpearmanCorrelation;
+pub use standard_error::StandardError;
 pub use standard_error_bands::{StandardErrorBands, StandardErrorBandsOutput};
 pub use starc_bands::{StarcBands, StarcBandsOutput};
 pub use stc::Stc;
@@ -313,6 +338,7 @@ pub use ttm_squeeze::{TtmSqueeze, TtmSqueezeOutput};
 pub use typical_price::TypicalPrice;
 pub use ulcer_index::UlcerIndex;
 pub use ultimate_oscillator::UltimateOscillator;
+pub use variance::Variance;
 pub use vertical_horizontal_filter::VerticalHorizontalFilter;
 pub use vidya::Vidya;
 pub use volty_stop::VoltyStop;