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.
kingchenc
b971e671b4