GCFT-0439: The Constraint Manifold — Broken-Phase Accessibility, Coherence Thresholds, and the Operator-Mixing Gate

GCFT-0439 formalizes the broken-phase threshold as a constraint-topology gate rather than a temperature switch. It defines an admissible constraint manifold M_c = {Xi | C[Xi] > C_c}, freezes rules for the symbolic coherence functional, and specifies when previously vanishing operator sectors may acquire nonzero matrix elements. The endpoint remains sealed: the full coherence functional, BRST operator basis, and finite broken-phase matching are still open.