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/macd.rs

@@ -21,6 +21,19 @@ pub struct MacdOutput {
 /// is seeded from the first `signal` raw MACD values, so the first full
 /// [`MacdOutput`] is emitted after `slow + signal − 1` inputs (assuming the
 /// slow EMA seeded by then).
+///
+/// # Example
+///
+/// ```
+/// use wickra_core::{Indicator, MacdIndicator};
+///
+/// let mut indicator = MacdIndicator::new(3, 6, 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 MacdIndicator {
     fast: Ema,