Improve README and refactor Winsock TCP handling

[papanda925]

Summary

• replace the minimal README with Japanese-first setup, usage, limitations, troubleshooting, and official references
• correct Winsock API declarations for VBA7 32-bit and 64-bit Office by using LongPtr for socket handles
• use the native four-parameter send signature
• close accepted client sockets after each request and guard all cleanup paths
• stop closing the workbook and the entire Excel application automatically
• add repository-level static validation and a Windows GitHub Actions workflow

Validation

• CP932 source round-trip validation passed
• VBA Sub / Function structure counts are balanced
• static checks confirm LongPtr handles, ws2_32.dll, and removal of automatic Excel shutdown
• GitHub Actions completed successfully
• Windows Excel runtime verification completed on 2026-09-02
• MainForMultiProcess, testHELLO, testElse, and testQUIT completed successfully

Scope

This remains a low-level TCP/Winsock learning sample. HTTP, WebSocket, and LLM API functionality are intentionally out of scope.