Restructure `int_bounds_info` / BC types for clarity

[sbryngelson]

`int_bounds_info` started as a simple beg/end integer pair but has grown into a full boundary-condition parameter struct. The name is now misleading, the internal naming is inconsistent, and the type conflates two distinct concerns. This issue tracks restructuring it for clarity.

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Current state

type int_bounds_info
    integer                             :: beg, end          ! BC type codes / MPI neighbor ranks
    real(wp)                            :: vb1, vb2, vb3     ! wall velocity at beg face (components 1,2,3)
    real(wp)                            :: ve1, ve2, ve3     ! wall velocity at end face (components 1,2,3)
    real(wp)                            :: pres_in, pres_out ! prescribed pressure (GRCBC)
    real(wp), dimension(3)              :: vel_in, vel_out   ! prescribed velocity (GRCBC)
    real(wp), dimension(num_fluids_max) :: alpha_rho_in, alpha_in
    logical                             :: grcbc_in, grcbc_out, grcbc_vel_out
    logical                             :: isothermal_in, isothermal_out
    real(wp)                            :: Twall_in, Twall_out
end type int_bounds_info

! Declared as three flat variables:
type(int_bounds_info) :: bc_x, bc_y, bc_z

Problems:

1. Misnamed — int_bounds_info implies integer index bounds; the type now holds full BC physics.
2. Inconsistent face naming — vb/ve (wall velocity) and _in/_out (everything else) both encode the same spatial concept (beg face vs end face), just with different conventions.
3. Two concerns in one type — the integer beg/end codes (MPI ranks / BC type selectors) are unrelated to the physical face parameters.
4. Flat bc_x/bc_y/bc_z — should be bc%x, bc%y, bc%z, consistent with every other x/y/z grouping in the codebase (an unused bc_xyz_info wrapper type already exists for this purpose).

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Proposed restructure

!> Physical parameters for one face of a domain boundary.
type bc_face_t
    real(wp)                            :: pres          ! prescribed pressure (GRCBC)
    real(wp), dimension(3)              :: vel           ! prescribed velocity (GRCBC)
    ! Kept as individual scalars (not an array) for GPU_DECLARE compatibility:
    real(wp)                            :: vel_wall_1    ! wall velocity, component 1
    real(wp)                            :: vel_wall_2    ! wall velocity, component 2
    real(wp)                            :: vel_wall_3    ! wall velocity, component 3
    real(wp), dimension(num_fluids_max) :: alpha_rho     ! partial densities (inflow)
    real(wp), dimension(num_fluids_max) :: alpha         ! volume fractions (inflow)
    real(wp)                            :: T_wall        ! wall temperature (isothermal BC)
    logical                             :: grcbc         ! GRCBC enabled
    logical                             :: grcbc_vel     ! GRCBC velocity outflow
    logical                             :: isothermal    ! isothermal wall
end type bc_face_t

!> Boundary condition info for one coordinate direction.
type bc_dir_t
    integer          :: beg, end
    type(bc_face_t)  :: beg_face
    type(bc_face_t)  :: end_face
end type bc_dir_t

!> All domain boundary conditions, replacing flat bc_x/bc_y/bc_z.
type bc_t
    type(bc_dir_t) :: x, y, z
end type bc_t

type(bc_t) :: bc

Old → new access:

Old	New
bc_x%beg	bc%x%beg
bc_x%vb1	bc%x%beg_face%vel_wall_1
bc_x%ve1	bc%x%end_face%vel_wall_1
bc_x%pres_in	bc%x%beg_face%pres
bc_x%pres_out	bc%x%end_face%pres
bc_x%vel_in	bc%x%beg_face%vel
bc_x%vel_out	bc%x%end_face%vel
bc_x%grcbc_in	bc%x%beg_face%grcbc
bc_x%grcbc_out	bc%x%end_face%grcbc
bc_x%grcbc_vel_out	bc%x%end_face%grcbc_vel
bc_x%isothermal_in	bc%x%beg_face%isothermal
bc_x%Twall_in	bc%x%beg_face%T_wall
bc_x%alpha_rho_in	bc%x%beg_face%alpha_rho

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Scope

Files that need updating:

• src/common/m_derived_types.fpp — type definitions (drop int_bounds_info, bc_xyz_info; add bc_face_t, bc_dir_t, bc_t)
• src/common/m_boundary_common.fpp — all field accesses (~50 sites)
• src/simulation/m_global_parameters.fpp — declarations and GPU_DECLARE macros
• src/simulation/m_start_up.fpp — GPU_UPDATE macros and initialization
• src/simulation/m_mpi_proxy.fpp — field names in MPI broadcast strings
• src/pre_process/m_global_parameters.fpp — declarations
• Any other sites using bc_x%, bc_y%, bc_z%

The GPU_DECLARE macros become e.g.:

$:GPU_DECLARE(create='[bc%x%beg_face%vel_wall_1, bc%x%beg_face%vel_wall_2, bc%x%beg_face%vel_wall_3, \
                        bc%x%end_face%vel_wall_1, ...]')

[sbryngelson]

Folded into #1427 as item 4.