E15: add a runnable doctest to every indicator type

Only two doctests existed in wickra-core; none of the 25 indicator
types carried a runnable rustdoc example.

Add an "# Example" doctest to every public indicator type (all 26,
including RollingVwap): construct the indicator and stream 80 inputs
through update, asserting a value is produced. The candle-input
indicators build valid OHLCV candles inline. cargo test --doc
-p wickra-core now runs 28 doctests, all passing; fmt and clippy clean.

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

@@ -9,6 +9,19 @@ use crate::traits::Indicator;
 /// is produced after `period + 1` inputs: the seed averages the first `period`
 /// gains and losses, and the first emitted RSI corresponds to the input at
 /// index `period`.
+///
+/// # Example
+///
+/// ```
+/// use wickra_core::{Indicator, Rsi};
+///
+/// let mut indicator = Rsi::new(3).unwrap();
+/// let mut last = None;
+/// for i in 0..80 {
+///     last = indicator.update(100.0 + f64::from(i));
+/// }
+/// assert!(last.is_some());
+/// ```
 #[derive(Debug, Clone)]
 pub struct Rsi {
     period: usize,