fix(core): use f64::midpoint and stable %2 to satisfy newer toolchains

Two unrelated newer-toolchain breakages bundled because they hit on the
same CI run and have the same shape (newer Rust got stricter about
patterns we used):

1. clippy 1.95 added the manual_midpoint lint which fires on every
   instance of (a + b) / 2.0 with a help suggesting f64::midpoint.
   CI runs with -D warnings so it became a hard error. Twelve sites
   were affected — three real call sites in src/ohlcv.rs (median_price),
   src/indicators/donchian.rs (DonchianOutput.middle),
   src/indicators/ease_of_movement.rs (mid), and
   src/indicators/super_trend.rs (hl2); plus eight test-helper
   Candle::new constructions across accelerator_oscillator,
   atr_trailing_stop, chaikin_volatility, chandelier_exit,
   chande_kroll_stop, choppiness_index, super_trend, true_range.
   All twelve switched to f64::midpoint (stable since Rust 1.85,
   our workspace MSRV).

2. usize::is_multiple_of is still unstable (rust-lang/rust#128101) and
   only stabilizes in Rust 1.87, but the MSRV CI job uses 1.85. The
   two call sites in bollinger.rs and sma.rs (added with the R7
   periodic-reseed tests) switched back to i % 2 == 0.

crates/wickra-core/src/ohlcv.rs

@@ -110,7 +110,7 @@ impl Candle {
     /// The mid price `(high + low) / 2`.
     #[inline]
     pub fn median_price(&self) -> f64 {
-        (self.high + self.low) / 2.0
+        f64::midpoint(self.high, self.low)
     }
 
     /// The weighted close `(high + low + 2*close) / 4`.