states.SoftReplace

Each tracklet's field becomes ``sum_j p[i, j] * detection.field[j]``, where ``p`` is an ``(N, M)`` row-stochastic transport plan. For ``p[i] = e_j`` (one-hot) this reduces to the hard :class:`~unitrack.states.Replace`. ``soft_assignment`` may be supplied at construction when the caller has already computed the plan; otherwise the plan is read at call time from ``match.soft_plan`` (attribute on :class:`~unitrack.data.MatchOutcome`), which :class:`~unitrack.assignment.Associate` attaches automatically when its assignment backend is a :class:`~unitrack.assignment.SoftAssignment`. Rows of the transport plan that sum to zero (every pair forbidden by an upstream gate) preserve the prior tracklet field rather than overwriting it with the all-zero blend that ``plan @ v`` would produce — that would silently destroy re-id embeddings for fully-gated tracklets.