Add beta Excel/VBA interface

[markleader]

Summary

Adds a beta/experimental Excel interface for CEA, intended for 64-bit Windows Excel/VBA users who need live worksheet formulas backed by the native CEA library. The interface is optional, disabled by default, and documented as a Windows 64-bit-only path; users who only need spreadsheet-formatted outputs are encouraged to use the Python API with Pandas and export to CSV/XLSX.

Changes

• Adds a new optional CEA_ENABLE_BIND_EXCEL CMake binding path, defaulting to OFF.
• Adds the native cea_excel.dll wrapper API, public Excel header, and smoke/API tests.
• Adds VBA modules for DLL loading, worksheet UDFs, helper functions, and diagnostics.
• Adds cea_template.xlsm as the starter macro-enabled workbook.
• Adds Excel interface docs under docs/source/interfaces/excel_api.rst.
• Updates installation/interface docs and binding README guidance for:
  • beta/experimental status
  • 64-bit Windows Excel/VBA-only support
  • Python + Pandas as the recommended workflow for spreadsheet exports
  • Excel DLL build and workbook setup steps

Compatibility / Numerical behavior

• No expected changes to numerical results
• Expected changes (explain and provide validation)

The Excel binding is additive and opt-in. Standard builds keep CEA_ENABLE_BIND_EXCEL=OFF, and the wrapper delegates solve behavior to the existing C/Fortran CEA APIs.