Internal workhorse called by kuramoto(). For each trimmed timepoint,
computes the global Kuramoto order parameter R(t):
Details
$$R(t) = \frac{1}{N} \left| \sum_{j=1}^{N} e^{i\phi_j(t)} \right|$$
The complex exponential is decomposed as \(\cos\phi + i\sin\phi\); the modulus is \(\sqrt{(\sum\cos)^2 + (\sum\sin)^2} / N\), avoiding any R complex-number allocation.
The first and last 10 timepoints are discarded (matching dyn_phase_lock()).