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/indicators/bollinger.rs

@@ -294,7 +294,7 @@ mod tests {
         let n_updates = 16 * period * 5;
         let mut last = None;
         for i in 0..n_updates {
-            let v = if i.is_multiple_of(2) { 1e6 } else { 1.0 };
+            let v = if i % 2 == 0 { 1e6 } else { 1.0 };
             last = bb.update(v);
             if window.len() == period {
                 window.pop_front();