bug: Homebrew v5.3.1 install crashes on startup — num_ib_patches_max missing from Fortran constants fallback

[sbryngelson]

Summary

Every MFC v5.3.1 Homebrew install crashes immediately on startup with:

RuntimeError: Fortran constant 'num_ib_patches_max' not found in m_constants.fpp.
Toolchain is out of sync with Fortran source.

This breaks mfc case.py for all users who installed via brew install mflowcode/mfc/mfc.

Root cause

Homebrew does not install src/ (only toolchain/, bin/, and examples/), so get_fortran_constants() in namelist_parser.py cannot find src/common/m_constants.fpp and falls back to _FALLBACK_CONSTANTS.

PR #1348 (Ib collisions, @danieljvickers) correctly split patch_ib onto its own Fortran bound by adding:

# toolchain/mfc/params/definitions.py
NIB = _fc("num_ib_patches_max")  # patch_ib (Fortran array bound)

But _FALLBACK_CONSTANTS in namelist_parser.py — the dict used when src/ is absent — was never updated to include "num_ib_patches_max". As a result, _fc() raised on every Homebrew startup.

The structural problem: _fc(name) and _FALLBACK_CONSTANTS were two separate places that had to stay in sync, with no enforcement.

Affected versions

• v5.3.1 (introduced by Ib collisions #1348, shipped in this release)
• v5.3.0 and earlier: not affected (NIB did not yet exist)

Fix

PR #1419 removes _FALLBACK_CONSTANTS entirely and changes _fc to require an inline default:

NIB = _fc("num_ib_patches_max", 50000)  # can't forget — default is right here

Going forward, any developer adding a new _fc() call must supply the fallback value at the call site. It is a required argument.

cc @danieljvickers