rheologicalfront.c: Rheological front tracking

Source under test: rheologicalfront.c

Reference-pinned test: tests/test_rheologicalfront.py::test_front_location_is_consistent_with_melt_fraction_profile

Anchor: Analytical self-consistency: the reported front index matches the phi_critical = 0.4 crossing of the melt-fraction profile, and the depth equals the surface radius minus the front radius.

Tolerance: Front index within one node of the recomputed crossing; rel 1e-6 on the depth identity.

Discrimination guards: Depth positivity and the mantle-depth bound; front pressure within the mantle range; the un-formed-front limit at t = 0 reports the core-mantle boundary by convention (pinned as the limit-input contract); the front migrates upward as the mantle solidifies bottom-up.

The rheological front marks the transition between the low- and high-viscosity regimes that PROTEUS reads as the dynamic front depth. Its location is recomputed independently from the melt-fraction profile and the geometry arrays of the same output.