fix: propagate exceptions from CustomThread to caller on join() (#45)

[tejassinghbhati]

Fixes CustomThread silently discarding exceptions from background threads.

• Captures exception in run() using sys.exc_info()
• Re-raises with original traceback in join()
• Successful return values unaffected

Closes #45

[SeaCelo]

Retargeted to mac-port per the macOS integration track: #51.
We are validating Mac/cross-platform changes in mac-port first, then merging mac-port -> main.