Refactor DDSPLIT: Extract counting operations into focused helper subroutines

[Copilot]

Description of the feature or the bug

The DDSPLIT subroutine was 3,728 lines—too large for effective maintenance. The file contained multiple distinct logical operations (element counting, boundary condition counting, statistics computation) that could be extracted into focused helpers.

Description of the changes

Extracted five counting/computation blocks into separate subroutines:

• ddsplit_count_elements.F (120 lines): Element type counting (solids, shells, beams) and numbering shifts
• ddsplit_count_bc_loads.F (109 lines): Boundary conditions (IBCL, IBFV) and loads (LLINK, IGRAV, IBVEL)
• ddsplit_count_thermal.F (95 lines): Thermal boundary conditions (convection, radiation, fluxes)
• ddsplit_count_constraints.F (121 lines): Rigid bodies (RBY, RBYM) and constraint elements (RBE2, RBE3)
• ddsplit_compute_stats.F (185 lines): Domain decomposition statistics and memory estimates (DDSTAT array)

Main file reduced from 3,728 to 3,562 lines. Each helper has a single responsibility, follows existing .F fixed-form conventions, and preserves exact functional behavior.

Original prompt

I would like to reduce the size of subroutine DDSPLIT in the file ddsplit.F
Please refactor this huge subroutine, extracting code snippet into subroutines

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