OBJ reader uses j as both triangle counter and vertex index

[sbryngelson]

Bug

In `src/common/m_model.fpp`, the OBJ file reader uses variable `j` as both the triangle counter and the third vertex index when parsing face lines. After `j` is incremented as a triangle counter, it is also used to index the third vertex, producing an off-by-one vertex reference.

File: `src/common/m_model.fpp`, lines 278-282

Impact: Triangulated OBJ meshes silently reference wrong vertex data for the third vertex of each triangle, corrupting immersed boundary geometry.

Severity: Critical — produces silently wrong geometry for OBJ-based immersed boundaries.

[sbryngelson]

Fix: #1178