F7: add NATR, StdDev, Ulcer Index and Historical Volatility

Completes the F7 family (Volatility) end to end:

- Rust core: natr.rs (ATR as a percentage of close), std_dev.rs
  (rolling population standard deviation), ulcer_index.rs (RMS of
  trailing-high drawdowns — downside-only risk), historical_volatility.rs
  (annualised sample stddev of log returns). Each with a full Indicator
  impl, runnable doctest and reference / constant-series / warmup /
  reset / batch==streaming tests.
- Python: PyNatr / PyStdDev / PyUlcerIndex / PyHistoricalVolatility
  PyO3 classes + module registration + .pyi stubs.
- Node: StdDevNode / UlcerIndexNode via the scalar macro, explicit
  NatrNode and HistoricalVolatilityNode; index.d.ts and index.js updated.
- WASM: WasmStdDev / WasmUlcerIndex / WasmHistoricalVolatility via the
  scalar macro, explicit WasmNatr.
- Wiki: Indicator-Natr/StdDev/UlcerIndex/HistoricalVolatility.md plus
  rows in Indicators-Overview.md and entries in Home.md.

cargo fmt + clippy (core/wickra/data/wasm/node) clean; 350 core tests,
25 data tests and 49 doctests green.

crates/wickra-core/src/lib.rs

@@ -45,10 +45,11 @@ pub mod indicators;
 pub use error::{Error, Result};
 pub use indicators::{
     Adx, AdxOutput, Aroon, AroonOscillator, AroonOutput, Atr, AwesomeOscillator, BollingerBands,
-    BollingerOutput, Cci, Cmo, Coppock, Dema, Donchian, DonchianOutput, Dpo, Ema, Hma, Kama,
-    Keltner, KeltnerOutput, MacdIndicator, MacdOutput, MassIndex, Mfi, Mom, Obv, Pmo, Ppo, Psar,
-    Roc, RollingVwap, Rsi, Sma, Smma, StochRsi, Stochastic, StochasticOutput, Tema, Trima, Trix,
-    Tsi, UltimateOscillator, Vortex, VortexOutput, Vwap, Vwma, WilliamsR, Wma, Zlema, T3,
+    BollingerOutput, Cci, Cmo, Coppock, Dema, Donchian, DonchianOutput, Dpo, Ema,
+    HistoricalVolatility, Hma, Kama, Keltner, KeltnerOutput, MacdIndicator, MacdOutput, MassIndex,
+    Mfi, Mom, Natr, Obv, Pmo, Ppo, Psar, Roc, RollingVwap, Rsi, Sma, Smma, StdDev, StochRsi,
+    Stochastic, StochasticOutput, Tema, Trima, Trix, Tsi, UlcerIndex, UltimateOscillator, Vortex,
+    VortexOutput, Vwap, Vwma, WilliamsR, Wma, Zlema, T3,
 };
 pub use ohlcv::{Candle, Tick};
 pub use traits::{BatchExt, Chain, Indicator};