FDS Source: CC_IBM, address back-CFACE search and mesh-interface cut-…

[marcosvanella]

…face divergence issue.

Here:

• Pick the back-CFACE aligned with the back-surface normal for EXPOSED THERMALLY_THICK surfaces.
• Exchange the back triangle normal so cross-mesh back-CFACEs resolve correctly.
• Use the geometry-triangle normal for CFACE radiation.
• Recompute flux-limited species and enthalpy transport across cut-faces at mesh interfaces.