test(family-10): cover cold paths flagged by codecov (#57)

Add tests that exercise the protective fallbacks reachable via flat or
zero-valued input:

- `CenterOfGravity::zero_window_uses_zero_fallback` — den == 0 branch.
- `EhlersStochastic::flat_window_emits_zero` — range == 0 branch.
- `Mama::flat_input_uses_phase_fallback` and the matching
  `SineWave` variant — `i1` collapses to zero on a constant series.
- `Fama::new_with_valid_limits_constructs_via_mama` exercises the
  `Ok(Self { inner: Mama::new(..)? })` arm that no other test reaches.

Three branches were genuinely unreachable by construction, so the dead
code is removed rather than masked with an attribute:

- `Mama` clamped `alpha > fast_limit` after the lower-bound clamp; the
  upper bound is implied by `delta_phase >= 1` and `alpha = fast / delta_phase`.
- `CyberneticCycle` had a `0.0` fallback after the warmup gate that the
  3-slot ring buffers preclude (`count >= 7` => all five `Some`s).
- `DecyclerOscillator` used a `let-else { return None }` over a pair of
  `Decycler::update` calls that always emit `Some` from the first bar.

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

@@ -109,6 +109,19 @@ mod tests {
         ));
     }
 
+    #[test]
+    fn new_with_valid_limits_constructs_via_mama() {
+        // `classic()` bypasses `new` by going through `Mama::classic`; this
+        // test exercises the happy-path `Ok(Self { inner: Mama::new(..)? })`
+        // arm so the `?` doesn't only collapse to the error path.
+        let mut fama = Fama::new(0.5, 0.05).expect("valid Mama limits");
+        assert_eq!(fama.limits(), (0.5, 0.05));
+        for i in 0..60 {
+            fama.update(100.0 + (f64::from(i) * 0.3).sin() * 5.0);
+        }
+        assert!(fama.value().is_some());
+    }
+
     #[test]
     fn accessors_and_metadata() {
         let mut fama = Fama::classic();