Share the global-parameters core across executables; generate MPI broadcasts; build hygiene (#1552)

Author: sbryngelson

.claude/rules/common-pitfalls.md

@@ -14,7 +14,7 @@ covered in `docs/documentation/contributing.md`.
   bubbles and IB. Read that routine for the current value rather than assuming one.
 - Riemann solvers: left state at `j`, right state at `j+1`.
 - All equation indices live in the `eqn_idx` struct (`eqn_idx_info` in
-  `m_derived_types.fpp`, populated in `m_global_parameters.fpp`): `%cont`, `%mom`, `%E`,
+  `m_derived_types.fpp`, populated by `s_initialize_eqn_idx` in `m_global_parameters_common.fpp`): `%cont`, `%mom`, `%E`,
   `%adv`, plus optional ranges (`%bub`, `%stress`, `%species`, `%B`, ...). The old
   `contxb`/`momxb` shorthands are gone. Index positions depend on `model_eqns` and
   enabled features — changing either moves ALL indices; never hard-code one.
@@ -42,21 +42,33 @@ covered in `docs/documentation/contributing.md`.
 - Adding one: `_r()` definition + `_nv()` `NAMELIST_VARS` registration in
   `toolchain/mfc/params/definitions.py`; `case_validator.py` only if physics-constrained
   (with a `PHYSICS_DOCS` entry). Fortran declarations and namelist bindings are
-  auto-generated at CMake configure time — re-run cmake (or `./mfc.sh build`) after editing.
+  auto-generated at build time (ninja-tracked custom command) — re-run cmake (or `./mfc.sh build`) after editing.
 - Still manual: derived-type `TYPE` member definitions in `src/common/m_derived_types.fpp`;
   default-value assignments in `s_assign_default_values_to_user_inputs`; the
   `CASE_OPT_EXTRA_LINES` literal in `toolchain/mfc/params/generators/fortran_gen.py` (covers `num_dims`,
   `num_vels`, `weno_polyn`, `muscl_polyn`, `weno_num_stencils`, `wenojs`);
   multi-variable declaration lines (`bc_x/y/z`, `x/y/z_domain`, `x/y/z_output`, post's
-  `G`); and MPI broadcast lists in `m_mpi_proxy`. Everything else — scalar declarations,
-  plain arrays (`FORTRAN_ARRAY_DIMS` table in `definitions.py`), derived-type namelist
-  declarations including `GPU_DECLARE` lines and Doxygen descs (`TYPED_DECLS` table in
-  `definitions.py`), and the simulation case-optimization declaration block — is
-  auto-generated at CMake configure time.
-  Gotcha: after editing a generator or table, force regen via `cmake_gen.py` into
-  `build/staging/*/` or simply reconfigure (`./mfc.sh build`) — cached builds compile
-  stale includes. Under `--case-optimization` the baked-in constants are dropped from the
+  `G`); and the MPI broadcast residue in `m_mpi_proxy` (computed variables that are not
+  namelist-bound: `m_glb`/`n_glb`/`p_glb`, `cfl_dt`, `bc_io`, and complex struct-member
+  array loops — these cannot be auto-generated and stay hand-listed). Everything else — scalar declarations, plain arrays (`FORTRAN_ARRAY_DIMS`
+  table in `definitions.py`), derived-type namelist declarations including `GPU_DECLARE`
+  lines and Doxygen descs (`TYPED_DECLS` table in `definitions.py`), the simulation
+  case-optimization declaration block, and the per-target MPI broadcast lists for all
+  namelist-registry scalars (`generated_bcast.fpp`) — is regenerated at build time by a
+  ninja-tracked custom command (editing `params/*.py` triggers regeneration automatically).
+  Gotcha: ADDING a new file under `toolchain/mfc/params/` needs one reconfigure
+  (the custom command's DEPENDS list is globbed at configure time). Under `--case-optimization` the baked-in constants are dropped from the
   namelist, so changing one needs a *rebuild*, not a case edit.
+- Shared-state pattern: namelist declarations (`#:include 'generated_decls.fpp'`), the
+  `eqn_idx`/`sys_size`/`b_size`/`tensor_size` state variables, and the common defaults
+  core all live in `src/common/m_global_parameters_common.fpp`. Each per-target
+  `m_global_parameters.fpp` does `use m_global_parameters_common` (default-public), so
+  `use m_global_parameters` continues to work for all downstream modules without change.
+  Sim-only declarations (GPU_DECLARE, Re_idx allocation) stay in
+  `m_global_parameters_common` behind `#ifdef MFC_SIMULATION`. Generated includes
+  (`generated_decls.fpp`, `generated_bcast.fpp`, `generated_case_opt_decls.fpp`) must exist for every target — the build
+  emits stubs where the content is sim-only, so a common file that includes one will
+  compile for pre/post too.
 - Runtime checks (`@:PROHIBIT`) go where they run: shared →
   `src/common/m_checker_common.fpp`; simulation-only → `src/simulation/m_checker.fpp`;
   pre/post-only → `src/{pre,post}_process/m_checker.fpp` (their `s_check_inputs` are