test(stochastic): cover classic/periods/metadata + naive_k flat branch

Codecov flagged 13 uncovered lines in
crates/wickra-core/src/indicators/stochastic.rs (file at 93.43%):

  - classic() convenience constructor (76-78) — every test passed
    explicit (k_period, d_period) to new
  - periods() const accessor (81-83) — never queried
  - warmup_period (170-172), name (178-180) Indicator-impl bodies —
    never queried
  - line 208 (`50.0` literal) inside the test-only naive_k helper's
    flat-range branch — k_matches_naive feeds an oscillating price
    series, so the helper's range == 0 path was dead

Add classic_periods_and_metadata test asserting Stochastic::classic()
has periods (14, 3), warmup_period 16 (= 14 + 3 - 1) and name
"Stochastic". Extend flat_range_yields_k_50 to also call naive_k on
the flat candle series and verify the helper returns Some(50.0) for
every index ≥ k_period - 1 — exercises line 208 without diluting the
production-code assertion.

stochastic.rs is now at 198/198 lines, no behavioural change.

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

@@ -220,6 +220,20 @@ mod tests {
         assert!(matches!(Stochastic::new(14, 0), Err(Error::PeriodZero)));
     }
 
+    /// Cover the `Stochastic::classic()` convenience constructor plus the
+    /// `periods()` const accessor and the Indicator-impl `warmup_period`
+    /// / `name` methods. Existing tests called `Stochastic::new(_, _)`
+    /// directly and never asked for the configured periods, warmup
+    /// length, or name.
+    #[test]
+    fn classic_periods_and_metadata() {
+        let s = Stochastic::classic();
+        assert_eq!(s.periods(), (14, 3));
+        // Warmup for the classic config: k_period + d_period - 1 = 14 + 3 - 1 = 16.
+        assert_eq!(s.warmup_period(), 16);
+        assert_eq!(s.name(), "Stochastic");
+    }
+
     #[test]
     fn close_at_high_yields_k_100() {
         let candles = vec![
@@ -252,6 +266,13 @@ mod tests {
             assert_relative_eq!(o.k, 50.0, epsilon = 1e-12);
             assert_relative_eq!(o.d, 50.0, epsilon = 1e-12);
         }
+        // Cross-check: the naive_k test helper must agree on the flat-range
+        // convention. The k_matches_naive test only feeds oscillating prices,
+        // so the helper's flat-range branch was never exercised.
+        let ks = naive_k(&candles, 14);
+        for k in ks.into_iter().skip(13) {
+            assert_relative_eq!(k.expect("ready after 14 inputs"), 50.0, epsilon = 1e-12);
+        }
     }
 
     #[test]