Add module-level @brief descriptions for all 16 undocumented modules

Doxygen uses the comment block directly before the module declaration
(not the @file header) to populate the Modules List page. These 16
modules only had file-level @Briefs, so they appeared with empty
descriptions on the namespace list pages.

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

Author: sbryngelson

src/common/m_chemistry.fpp

@@ -6,6 +6,7 @@
 #:include 'macros.fpp'
 #:include 'case.fpp'
 
+!> @brief Multi-species chemistry interface for thermodynamic properties, reaction rates, and transport coefficients
 module m_chemistry
 
     use m_thermochem, only: &

src/common/m_constants.fpp

@@ -2,6 +2,7 @@
 !! @file
 !! @brief Contains constant values used throughout the code(s).
 
+!> @brief Compile-time constant parameters: default values, tolerances, and physical constants
 module m_constants
 
     use m_precision_select

src/common/m_delay_file_access.f90

@@ -2,6 +2,7 @@
 !! @file
 !! @brief Contains module m_delay_file_access
 
+!> @brief Rank-staggered file access delays to prevent I/O contention on parallel file systems
 module m_delay_file_access
     use m_precision_select
     implicit none

src/common/m_finite_differences.fpp

@@ -4,6 +4,7 @@
 
 #:include 'macros.fpp'
 
+!> @brief Finite difference operators for computing divergence of velocity fields
 module m_finite_differences
 
     use m_global_parameters

src/common/m_helper.fpp

@@ -5,6 +5,7 @@
 !! @file
 !! @brief Contains module m_helper
 
+!> @brief Utility routines for bubble model setup, coordinate transforms, array sampling, and special functions
 module m_helper
 
     use m_derived_types        !< Definitions of the derived types

src/common/m_helper_basic.fpp

@@ -4,6 +4,7 @@
 
 #:include 'macros.fpp'
 
+!> @brief Basic floating-point utilities: approximate equality, default detection, and coordinate bounds
 module m_helper_basic
 
     use m_derived_types        !< Definitions of the derived types

src/common/m_model.fpp

@@ -5,6 +5,7 @@
 
 #:include 'macros.fpp'
 
+!> @brief Binary STL file reader and processor for immersed boundary geometry
 module m_model
 
     use m_helper

src/common/m_nvtx.f90

@@ -2,6 +2,7 @@
 !! @file
 !! @brief Contains module m_nvtx
 
+!> @brief NVIDIA NVTX profiling API bindings for GPU performance instrumentation
 module m_nvtx
 
     use iso_c_binding

src/common/m_phase_change.fpp

@@ -5,6 +5,7 @@
 #:include 'case.fpp'
 #:include 'macros.fpp'
 
+!> @brief Phase transition relaxation solvers for liquid-vapor flows with cavitation and boiling
 module m_phase_change
 
 #ifndef MFC_POST_PROCESS

src/pre_process/m_assign_variables.fpp

@@ -5,6 +5,7 @@
 #:include 'case.fpp'
 #:include 'macros.fpp'
 
+!> @brief Assigns initial primitive variables to computational cells based on patch geometry
 module m_assign_variables
 
     use m_derived_types         ! Definitions of the derived types