Fix: eliminate Windows PID race condition by replacing concorekill.bat overwrite mechanism with safe PID registry (#391)

[GaneshPatil7517]

Problem

On Windows, concore.py wrote a single PID to concorekill.bat at module import time
using write mode ("w"). When multiple Python nodes launched simultaneously via
makestudy, each node overwrote the file — only the last node's PID survived.

This caused:

• Race condition: N-1 of N nodes became unkillable via concorekill.bat
• Stale PID danger: after a crash, the leftover file could contain a PID reassigned
  to an unrelated process, risking taskkill terminating the wrong program
• No cleanup: PID file persisted after node exit

Solution

Replaced the single-PID overwrite with an append-based PID registry system:

Before	After
concorekill.bat with one hardcoded taskkill line	concorekill_pids.txt registry (one PID per line, append mode)
Last writer wins	All nodes coexist safely
No exit cleanup	atexit handler removes current PID on shutdown
Blind taskkill	Generated concorekill.bat validates each PID via tasklist before killing

Key changes in concore.py:

• _register_pid() — appends current PID to concorekill_pids.txt
• _cleanup_pid() — removes current PID on exit; deletes both files when last node exits
• _write_kill_script() — generates concorekill.bat that checks each PID is a running Python process before issuing taskkill

Backward compatibility:

Users still run concorekill.bat exactly as before — no workflow change required.
Only Python layer modified; C++, MATLAB, and Verilog implementations are untouched.

Tests

Added 9 tests in TestPidRegistry:

• Registration creates file and appends (not overwrites)
• Cleanup removes only current PID, preserves others
• Last PID cleanup deletes both registry and kill script
• Missing registry handled gracefully (no crash)
• Kill script contains validation logic (tasklist + taskkill)
• Multi-node simulation (3+ PIDs coexist)
• Windows import-time registration

All 9 new tests pass. All pre-existing tests unaffected.

Fixes

Closes #391