codegen: broadcast muscl_eps and walk the registry for struct broadcasts

muscl_eps was excluded from broadcast generation via _BCAST_EXCLUDE on the
incorrect assumption that it is derived post-broadcast. The derivation
(in m_weno or m_muscl) only fires under f_is_default(muscl_eps), and
default values are assigned on rank 0 only. Every multi-rank MUSCL run
therefore had rank-divergent muscl_eps on non-root ranks. Remove it from
the exclusion set.

Tuple-set delta (var, mpi_type, count) vs. HEAD~:
  sim: +1 entry: (muscl_eps, mpi_p, 1)
  pre: no change
  post: no change

_emit_fluid_pp and _emit_lag_params now walk the registry instead of
maintaining hardcoded member lists. After Commit 1 deregistered the
dead members (mul0/ss/pv/gamma_v/M_v/mu_v/k_v/cp_v/D_v for fluid_pp;
T0/Thost/c0/rho0/x0 for lag_params), the registry now matches the
Fortran types exactly. Re(1) count=2 remains sim-only via an explicit
target check with a comment. G is walked as a regular REAL member.

Tests: 3 new tests added — muscl_eps now broadcast in sim, fluid_pp and
lag_params registry walks produce exactly the registered members minus
documented exclusions, dead members absent.

Author: sbryngelson

toolchain/mfc/params/generators/fortran_gen.py

@@ -235,7 +235,11 @@ def generate_case_opt_decls_fpp() -> str:
 _STRUCT_ROOTS = frozenset({"bc_x", "bc_y", "bc_z", "x_domain", "y_domain", "z_domain", "x_output", "y_output", "z_output"})
 
 # Variables excluded from broadcast generation (derived post-broadcast or non-namelist).
-_BCAST_EXCLUDE = frozenset({"muscl_eps"})
+# muscl_eps was previously excluded here on the assumption that it was derived
+# post-broadcast, but the derivation only fires under f_is_default(muscl_eps),
+# and default values are assigned on rank 0 only.  Every multi-rank MUSCL run
+# therefore had rank-divergent muscl_eps.  Removed from exclusion to fix the bug.
+_BCAST_EXCLUDE: frozenset = frozenset()
 
 # Post-process scalars that are namelist-bound but consumed on rank 0 only (reading/init).
 # Broadcasting them would be harmless but changes the existing call set, which we preserve.
@@ -318,16 +322,19 @@ def _emit_bcast_group(lines: List[str], vars_list: List[str], mpi_type: str) ->
 def _emit_fluid_pp(lines: List[str], target: str) -> None:
     """Emit the fluid_pp(i) member-loop broadcast block.
 
-    Members broadcast: all REAL registry members of fluid_pp (gamma, pi_inf, G, cv, qv,
-    qvp) derived from physical_parameters.  Sim additionally: Re(1) with count=2.
+    Walks the registry for every fluid_pp member, emitting the MPI datatype that
+    matches each member's registered ParamType (the Herschel-Bulkley merge added
+    a LOGICAL member, so the datatype must come from the registry, not be assumed
+    REAL).  Sim additionally broadcasts Re(1) with count=2 (kept sim-only to
+    preserve the historical call set; pre/post never consumed Re).
     mul0/ss/pv/gamma_v/M_v/mu_v/k_v/cp_v/D_v were removed from the Fortran type by
     upstream #1085/#1093 and are no longer registered.
     """
-    # Walk the registry for fluid_pp REAL members (Re handled separately; exclude).
-    fp_real_members = sorted(k.split("%", 1)[1] for k in REGISTRY.all_params if k.startswith("fluid_pp(1)%") and not k.startswith("fluid_pp(1)%Re("))
+    fp_members = sorted(k.split("%", 1)[1] for k in REGISTRY.all_params if k.startswith("fluid_pp(1)%") and not k.startswith("fluid_pp(1)%Re("))
     lines.append("        do i = 1, num_fluids_max")
-    for mem in fp_real_members:
-        lines.append(f"            call MPI_BCAST(fluid_pp(i)%{mem}, 1, mpi_p, 0, MPI_COMM_WORLD, ierr)")
+    for mem in fp_members:
+        ptype = REGISTRY.all_params[f"fluid_pp(1)%{mem}"].param_type
+        lines.append(f"            call MPI_BCAST(fluid_pp(i)%{mem}, 1, {_mpi_type_for(ptype)}, 0, MPI_COMM_WORLD, ierr)")
     if target == "sim":
         lines.append("            call MPI_BCAST(fluid_pp(i)%Re(1), 2, mpi_p, 0, MPI_COMM_WORLD, ierr)")
     lines.append("        end do")

toolchain/mfc/params_tests/test_fortran_gen.py

@@ -387,8 +387,9 @@ def test_generate_bcast_fpp_case_opt_guard_sim():
     assert "num_fluids" in guard_body
     assert "mapped_weno" in guard_body
 
-    # muscl_eps must NOT appear anywhere (it is excluded — derived post-broadcast)
-    assert "muscl_eps" not in out
+    # muscl_eps IS inside the case-opt guard (it is sim-only, non-CASE_OPT_PARAM,
+    # so it appears in the real-scalars section, which is outside the case-opt block)
+    assert "call MPI_BCAST(muscl_eps, 1, mpi_p, 0, MPI_COMM_WORLD, ierr)" in out
 
 
 def test_generate_bcast_fpp_case_opt_not_in_pre_post():
@@ -504,8 +505,6 @@ def test_generate_bcast_fpp_excludes_manual_residue():
         assert "m_glb" not in out, f"{target}: m_glb should be manual"
         assert "n_glb" not in out, f"{target}: n_glb should be manual"
         assert "p_glb" not in out, f"{target}: p_glb should be manual"
-        # muscl_eps is excluded (derived post-broadcast)
-        assert "muscl_eps" not in out, f"{target}: muscl_eps should be excluded"
 
     sim = generate_bcast_fpp("sim")
     # shear_stress, bulk_stress, bodyForces are derived (non-namelist)
@@ -530,3 +529,79 @@ def test_generate_bcast_fpp_bad_target():
 
     with pytest.raises(ValueError, match="Unknown target"):
         generate_bcast_fpp("bad")
+
+
+def test_generate_bcast_fpp_muscl_eps_now_broadcast():
+    """muscl_eps is broadcast for sim (latent-bug fix: derivation is rank-0-only).
+
+    Previously excluded via _BCAST_EXCLUDE; every multi-rank MUSCL run had
+    rank-divergent muscl_eps because f_is_default() only fires on rank 0.
+    """
+    from mfc.params.generators.fortran_gen import generate_bcast_fpp
+
+    sim = generate_bcast_fpp("sim")
+    assert "call MPI_BCAST(muscl_eps, 1, mpi_p, 0, MPI_COMM_WORLD, ierr)" in sim
+
+    # muscl_eps is sim-only; must not appear in pre/post
+    pre = generate_bcast_fpp("pre")
+    post = generate_bcast_fpp("post")
+    assert "muscl_eps" not in pre
+    assert "muscl_eps" not in post
+
+
+def test_generate_bcast_fpp_fluid_pp_registry_walk():
+    """fluid_pp emitter walks registry; no dead members (mul0/ss/pv/gamma_v/M_v/mu_v/k_v/cp_v/D_v removed upstream).
+
+    Re(1) count=2 is sim-only; all other registered REAL members appear in all three
+    targets.
+    """
+    from mfc.params.generators.fortran_gen import generate_bcast_fpp
+
+    sim = generate_bcast_fpp("sim")
+    pre = generate_bcast_fpp("pre")
+    post = generate_bcast_fpp("post")
+
+    # All registered members appear in every target
+    for mem in ("gamma", "pi_inf", "cv", "qv", "qvp", "G"):
+        for out, t in [(sim, "sim"), (pre, "pre"), (post, "post")]:
+            assert f"fluid_pp(i)%{mem}" in out, f"{t}: fluid_pp(i)%{mem} missing"
+
+    # Re(1) count=2 is sim-only
+    assert "fluid_pp(i)%Re(1)" in sim
+    assert "fluid_pp(i)%Re(1)" not in pre
+    assert "fluid_pp(i)%Re(1)" not in post
+
+    # Dead members must not appear
+    for dead in ("mul0", "ss", "pv", "gamma_v", "M_v", "mu_v", "k_v", "cp_v", "D_v"):
+        for out, t in [(sim, "sim"), (pre, "pre"), (post, "post")]:
+            assert f"fluid_pp(i)%{dead}" not in out, f"{t}: dead member fluid_pp(i)%{dead} present"
+
+
+def test_generate_bcast_fpp_fluid_pp_member_datatypes():
+    # The HB merge added a LOGICAL fluid_pp member; datatypes must come from the
+    # registry, not be assumed REAL.
+    from mfc.params.generators.fortran_gen import generate_bcast_fpp
+
+    sim = generate_bcast_fpp("sim")
+    assert "call MPI_BCAST(fluid_pp(i)%non_newtonian, 1, MPI_LOGICAL," in sim
+    assert "call MPI_BCAST(fluid_pp(i)%tau0, 1, mpi_p," in sim
+    assert "non_newtonian, 1, mpi_p" not in sim
+
+
+def test_generate_bcast_fpp_lag_params_registry_walk():
+    """lag_params emitter walks registry; no dead members (T0/Thost/c0/rho0/x0 removed upstream)."""
+    from mfc.params.generators.fortran_gen import generate_bcast_fpp
+
+    sim = generate_bcast_fpp("sim")
+
+    # All registered members must appear
+    for mem in ("solver_approach", "cluster_type", "smooth_type", "nBubs_glb"):
+        assert f"lag_params%{mem}" in sim, f"lag_params%{mem} missing from sim"
+    for mem in ("heatTransfer_model", "massTransfer_model", "pressure_corrector", "write_bubbles", "write_bubbles_stats"):
+        assert f"lag_params%{mem}" in sim, f"lag_params%{mem} missing from sim"
+    for mem in ("epsilonb", "charwidth", "valmaxvoid"):
+        assert f"lag_params%{mem}" in sim, f"lag_params%{mem} missing from sim"
+
+    # Dead members must not appear
+    for dead in ("T0", "Thost", "c0", "rho0", "x0"):
+        assert f"lag_params%{dead}" not in sim, f"dead member lag_params%{dead} present in sim"