docs: update parameter-addition rule from 4-location to 2-location

Fortran decls and namelist bindings are now auto-generated. Simple scalar
params only need definitions.py (_r + _nv) + cmake reconfigure.

Author: sbryngelson

.claude/rules/common-pitfalls.md

@@ -59,7 +59,7 @@ Before submitting a PR:
 - [ ] `./mfc.sh precheck -j 8` (5 CI lint checks)
 - [ ] `./mfc.sh build -j 8` (compiles)
 - [ ] `./mfc.sh test --only <relevant> -j 8` (tests pass)
-- [ ] If adding parameters: all 4 locations updated
+- [ ] If adding parameters: definitions.py (_r + _nv) updated; cmake reconfigured; case_validator.py if constraints
 - [ ] If modifying `src/common/`: all three targets tested
 - [ ] If changing output: golden files regenerated for affected tests
 - [ ] One logical change per commit

.claude/rules/parameter-system.md

@@ -23,20 +23,22 @@ MFC has ~3,400 simulation parameters defined in Python and read by Fortran via n
    - Reads `&user_inputs` namelist
    - Each parameter must be declared in the namelist statement
 
-## Adding a New Parameter (4-location checklist)
+## Adding a New Parameter (2-location checklist)
 
-YOU MUST update the first 3 locations. Missing any causes silent failures or compile errors.
-Location 4 is required only if the parameter has physics constraints.
+Fortran declarations and namelist bindings are now auto-generated from definitions.py
+at CMake configure time — no manual Fortran edits needed for simple scalar parameters.
 
-1. **`toolchain/mfc/params/definitions.py`**: Add parameter with type, default, constraints
-2. **`src/*/m_global_parameters.fpp`**: Declare the Fortran variable in the relevant
-   target(s). If the param is used by simulation only, add it there. If shared, add to
-   all three targets' m_global_parameters.fpp.
-3. **`src/*/m_start_up.fpp`**: Add to the Fortran `namelist` declaration in the relevant
-   target(s).
-4. **`toolchain/mfc/case_validator.py`**: Add validation rules if the parameter has
+1. **`toolchain/mfc/params/definitions.py`**: Add parameter with `_r()` (type, default,
+   constraints) AND add it to `NAMELIST_VARS` via `_nv()` for the relevant target(s).
+   After editing, re-run cmake (or `./mfc.sh build`) to regenerate the Fortran includes.
+2. **`toolchain/mfc/case_validator.py`**: Add validation rules if the parameter has
    physics constraints. Include `PHYSICS_DOCS` entry with title, category, explanation.
 
+**Exceptions — still require manual Fortran edits:**
+- Array variables (e.g. `logical, dimension(num_fluids_max)`) → declare in `src/*/m_global_parameters.fpp`
+- Derived-type members (`fluid_pp%attr`, `patch_icpp(i)%attr`) → declare in the relevant derived type
+- Case-optimization parameters → add to `CASE_OPT_PARAMS` and the `#:else` block in `src/simulation/m_global_parameters.fpp`
+
 ## Case Files
 - Case files are Python scripts (`.py`) that define a dict of parameters
 - Validated with `./mfc.sh validate case.py`

CLAUDE.md

@@ -139,11 +139,12 @@ NEVER use `stop` or `error stop`. Use `call s_mpi_abort()` or `@:PROHIBIT()`/`@:
 NEVER use `goto`, `COMMON` blocks, or global `save` variables.
 
 Every `@:ALLOCATE(...)` MUST have a matching `@:DEALLOCATE(...)`.
-Every new parameter MUST be added in at least 3 places (4 if it has constraints):
-  1. `toolchain/mfc/params/definitions.py` (parameter definition)
-  2. Fortran variable declaration in `src/*/m_global_parameters.fpp`
-  3. Fortran namelist in `src/*/m_start_up.fpp` (namelist binding)
-  4. `toolchain/mfc/case_validator.py` (only if parameter has physics constraints)
+Every new parameter MUST be added in at least 2 places (3 if it has constraints):
+  1. `toolchain/mfc/params/definitions.py` (parameter definition + NAMELIST_VARS target set)
+  2. `toolchain/mfc/case_validator.py` (only if parameter has physics constraints)
+  Note: Fortran declarations and namelist bindings are auto-generated from definitions.py
+  at CMake configure time. Simple scalars need no manual Fortran edits. Array/derived-type
+  variables still require a manual declaration in `src/*/m_global_parameters.fpp`.
 
 Changes to `src/common/` affect ALL three executables. Test comprehensively.