Explorer hangs when the SV-COMP worker thread raises #62

If the SV-COMP worker thread (run_sv_comp) raises, the explorer process keeps running because the HTTP server thread is still alive. The run hangs until the time limit and becomes a TIMEOUT instead of an immediate UNKNOWN.

Seen in the full Java run (2026-10-15) with:

• ValueError: embedded null byte from a \0 char input (Char input \0 crashes the explorer (embedded null byte) #61)
• RecursionError while building the execution tree from long traces (Explorer fails on very large traces (RecursionError building the execution tree) #59)

Expected: any uncaught exception in the worker prints [VERDICT <prp>] == DONT-KNOW, logs the traceback, and exits.