Add 10 pairwise stat-arb indicators to Price Statistics (#154)

Adds ten pairwise `(f64, f64)` indicators to the **Price Statistics** family, completing the A1 stat-arb expansion block.

## Indicators

**Scalar output:**
- **RollingCorrelation** — rolling Pearson correlation of period-over-period *returns* (distinct from level-based `PearsonCorrelation`).
- **RollingCovariance** — rolling covariance of returns.
- **OuHalfLife** — Ornstein–Uhlenbeck half-life of mean reversion of the spread `a − b`.
- **SpreadHurst** — Hurst exponent of the spread (variance-of-lagged-differences fit) for regime detection.
- **DistanceSsd** — Gatev sum-of-squared-deviations between two start-normalised series.
- **BetaNeutralSpread** — rolling OLS regression residual `a − (α + β·b)`.
- **VarianceRatio** — Lo–MacKinlay variance-ratio test on the spread (two params: `period`, `q`).
- **GrangerCausality** — F-statistic for whether `b` predicts `a` (two params: `period`, `lag`).

**Struct output (custom bindings):**
- **KalmanHedgeRatio** — dynamic hedge ratio via a Kalman filter → `{ hedgeRatio, intercept, spread }`.
- **SpreadBollingerBands** — Bollinger bands on the spread → `{ middle, upper, lower, percentB }`.

## Notes
- No new traits or input families: all use the native `Indicator<Input = (f64, f64)>` (precedent `Beta`, `Cointegration`).
- Adds `Error::InvalidParameter` for floating-point constructor parameters (Kalman `delta`/`observation_var`, `num_std`).
- Full Python/Node/WASM bindings; the two struct-output indicators are hand-written, the rest use the pair macros.
- Indicator count 315 → 325; README, family rows, `__init__`, fuzz target, and CHANGELOG updated.

## Verification
- `cargo test --workspace --all-features` — green (2676 core lib + 308 doc).
- `cargo clippy --workspace --all-targets --all-features -- -D warnings` — clean.
- Node: `npm run build && npm test` — 410 passing (`index.d.ts`/`index.js` regenerated).
- Python: `pytest` — 684 passing.

CHANGELOG.md

@@ -8,6 +8,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- **Spread Bollinger Bands** — Bollinger bands on the spread of two series for pairs mean-reversion (`SPREAD_BOLLINGER_BANDS`).
+- **Kalman Hedge Ratio** — Kalman-filter dynamic hedge ratio and spread between two series (`KALMAN_HEDGE_RATIO`).
+- **Granger Causality** — Granger causality F-statistic measuring whether one series predicts another (`GRANGER_CAUSALITY`).
+- **Variance Ratio** — Lo-MacKinlay variance-ratio test on the spread of two series (`VARIANCE_RATIO`).
+- **Beta-Neutral Spread** — beta-neutral spread: the rolling OLS regression residual of two series (`BETA_NEUTRAL_SPREAD`).
+- **Distance SSD** — Gatev sum-of-squared-deviations distance between two normalised series (`DISTANCE_SSD`).
+- **Spread Hurst** — Hurst exponent of the spread of two series for regime detection (`SPREAD_HURST`).
+- **OU Half-Life** — Ornstein-Uhlenbeck half-life of mean reversion for the spread of two series (`OU_HALF_LIFE`).
+- **Rolling Covariance** — rolling covariance of the period-over-period returns of two series (`ROLLING_COVARIANCE`).
+- **Rolling Correlation** — rolling Pearson correlation of the period-over-period returns of two series (`ROLLING_CORRELATION`).
 
 - **Market Breadth family** — a new indicator family built on a new
   `CrossSection` input type that carries the per-symbol state of an entire