nx_gm_xpbd_gate.nx

nx_gm_xpbd_gate.nx -- GATE for PG29's cloth solver (gm_xpbd_step in nx_nxa_garment.nx). THE DONE-RULE WAS PRE-DECLARED ON THE PLAN before this gate existed (procgen.plan PG29): "Done when a dropped sheet folds under gravity with area conserved within a derived bound and a planted stiffness change measurably changes the fold." Every tooth below is one clause of that sentence, plus the anti-vacuity controls the estate's gate law demands. Fixture: a ring-grid sheet built by the SAME gm_edges_build the solver uses, pinned at its top ring -- the garment's own GPIN contract shape. license_tier: ORIGINAL No hw writes (Rule 26).