Frame: gate fold refinement by Jacobian

README.md

@@ -180,13 +180,14 @@ computes the discrete lens Jacobian
 `J = Omega_source / Omega_image`.  Both signed solid angles use
 `2 atan2(dot(a, cross(b,c)), 1 + dot(a,b) + dot(b,c) + dot(c,a))`, with the
 ordered camera directions for `Omega_image` and their traced infinity
-directions for `Omega_source`.  `abs(J) < --refine-jacobian-min` requests a
-split near a critical curve.  The signed-area calculation retains the sign of
-`J`, but sign changes across adjacent triangles are not currently a refinement
-trigger; this makes it possible to evaluate the `abs(J)` criterion separately.
-The `1e-3` default is deliberately provisional and should be tuned with the
-small Schwarzschild refinement diagnostic before being treated as a production
-threshold.
+directions for `Omega_source`.  A J-driven split requires **both** a shared
+image edge whose incident triangles have opposite nonzero signs of `J` and
+`min(abs(J_left), abs(J_right)) < --refine-jacobian-min`.  It then requests
+that shared edge on both leaves.  Thus `|J|` bounds the fold selection instead
+of widening it as a standalone critical-curve band.  A negative sign is
+physical parity and is retained.  The `1e-3` default is deliberately
+provisional and should be tuned with the small Schwarzschild refinement
+diagnostic before being treated as a production threshold.
 
 For a short Schwarzschild diagnostic that permits at most one actual split
 generation, for example: