Fix file headers, add non-Newtonian tests, re-merge upstream

Fix .f90 -> .fpp in Doxygen headers for m_hb_function and m_re_visc.
Add shear-thinning and Bingham regression tests to CI test suite.
Re-merge with upstream/master after ruff PR (#1300) landed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sbryngelson

src/simulation/m_hb_function.fpp

@@ -1,5 +1,5 @@
 !>
-!! @file m_hb_function.f90
+!! @file m_hb_function.fpp
 !! @brief Contains module m_hb_function
 
 #:include 'macros.fpp'

src/simulation/m_re_visc.fpp

@@ -1,5 +1,5 @@
 !>
-!! @file m_re_visc.f90
+!! @file m_re_visc.fpp
 !! @brief Contains module m_re_visc
 
 #:include 'macros.fpp'

toolchain/mfc/test/cases.py

@@ -401,6 +401,28 @@ def alter_num_fluids(dimInfo):
 
                 stack.pop()
 
+            if num_fluids == 1:
+                # Non-Newtonian viscosity (Herschel-Bulkley)
+                stack.push(
+                    "Non-Newtonian",
+                    {
+                        "viscous": "T",
+                        "dt": 1e-11,
+                        "patch_icpp(1)%vel(1)": 1.0,
+                        "fluid_pp(1)%Re(1)": 10.0,
+                        "fluid_pp(1)%non_newtonian": "T",
+                        "fluid_pp(1)%tau0": 0.0,
+                        "fluid_pp(1)%K": 0.1,
+                        "fluid_pp(1)%nn": 0.5,
+                        "fluid_pp(1)%hb_m": 1000.0,
+                        "fluid_pp(1)%mu_min": 1e-4,
+                        "fluid_pp(1)%mu_max": 10.0,
+                    },
+                )
+                cases.append(define_case_d(stack, "shear-thinning", {}))
+                cases.append(define_case_d(stack, "Bingham", {"fluid_pp(1)%tau0": 0.1, "fluid_pp(1)%nn": 1.0, "fluid_pp(1)%K": 0.01}))
+                stack.pop()
+
             if num_fluids == 2:
                 stack.push(
                     "Viscous",