Add build-flag compatibility checks to Python case validator

Move three Fortran @:PROHIBIT checks that test case parameters against
build configuration flags into case_validator.py, so they fire before
any binary is invoked rather than at Fortran runtime:

- parallel_io = T requires --mpi (all stages, via check_build_flags)
- precision = 2 requires no --single (post_process, in check_output_format)
- 3D cylindrical + p > 0 requires no --single (simulation only,
  via check_geometry_precision_simulation)

CFG() from state.py carries the active build config (mpi, single, mixed).
During standalone './mfc.sh validate', CFG() holds defaults (mpi=T,
single=F) so these checks do not produce false positives.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sbryngelson

toolchain/mfc/case_validator.py

@@ -19,6 +19,7 @@
 from functools import lru_cache
 from .common import MFCException
 from .params.definitions import CONSTRAINTS
+from .state import CFG
 
 
 # Physics documentation for check methods.
@@ -1794,6 +1795,10 @@ def check_output_format(self):
         if precision is not None:
             self.prohibit(precision not in [1, 2],
                          "precision must be 1 or 2")
+            self.prohibit(
+                precision == 2 and CFG().single,
+                "precision = 2 (double output) requires MFC built without --single"
+            )
 
     def check_vorticity(self):
         """Checks vorticity parameters (post-process)"""
@@ -2330,13 +2335,41 @@ def check_velocity_components(self):
                     self.prohibit(vel3 != 0,
                                  f"patch_icpp({i})%vel(3) = {vel3} but p = 0 (1D/2D simulation)")
 
+    # ===================================================================
+    # Build-Flag Compatibility Checks
+    # ===================================================================
+
+    def check_build_flags(self):
+        """Checks case parameters against the active build configuration.
+
+        These catch incompatibilities between case settings and how the
+        MFC binaries were compiled (--mpi/--no-mpi, --single, etc.)
+        before any binary is invoked.
+        """
+        parallel_io = self.get('parallel_io', 'F') == 'T'
+        self.prohibit(
+            parallel_io and not CFG().mpi,
+            "parallel_io = T requires MFC built with --mpi"
+        )
+
+    def check_geometry_precision_simulation(self):
+        """Checks that 3D cylindrical geometry is not used with --single builds."""
+        cyl_coord = self.get('cyl_coord', 'F') == 'T'
+        p = self.get('p', 0)
+        self.prohibit(
+            CFG().single and cyl_coord and p is not None and p > 0,
+            "Fully 3D cylindrical geometry (cyl_coord = T, p > 0) is not supported "
+            "in single precision (--single)"
+        )
+
     # ===================================================================
     # Main Validation Entry Points
     # ===================================================================
 
     def validate_common(self):
         """Validate parameters common to all stages"""
         self.check_parameter_types()  # Type validation first
+        self.check_build_flags()
         self.check_simulation_domain()
         self.check_domain_bounds()
         self.check_model_eqns_and_num_fluids()
@@ -2359,6 +2392,7 @@ def validate_common(self):
     def validate_simulation(self):
         """Validate simulation-specific parameters"""
         self.validate_common()
+        self.check_geometry_precision_simulation()
         self.check_finite_difference()
         self.check_time_stepping()
         self.check_riemann_solver()

toolchain/mfc/lint_docs.py

@@ -423,6 +423,8 @@ def check_physics_docs_coverage(repo_root: Path) -> list[str]:
         "check_output_format",       # output format selection
         "check_restart",             # restart file logistics
         "check_parallel_io_pre_process",  # parallel I/O settings
+        "check_build_flags",              # build-flag compatibility (no physics meaning)
+        "check_geometry_precision_simulation",  # build-flag compatibility (no physics meaning)
         "check_misc_pre_process",    # miscellaneous pre-process flags
         "check_bc_patches",          # boundary patch geometry
         "check_grid_stretching",     # grid stretching parameters