CI: DF crashes with 'free(): invalid pointer' at quit after test suite passes #5947

The Linux gcc-11 test leg is intermittently failing with a heap-corruption crash at DF shutdown, after the test suite reports all tests passed. The subsequent DF launch in the same attempt then aborts at startup.

Observed on two unrelated branches:

• https://github.com/DFHack/dfhack/actions/runs/35271091388/job/105371147488 (suspendmanager-machine-collapse)
• https://github.com/DFHack/dfhack/actions/runs/35269070516 (add-tests, gcc-11 + gcc-12 + windows legs all failed in that run)

Tail of stderr.log from both runs is identical:

Tests completed in 10920 ms:
  27/27 tests passed
  185/185 checks passed
Invoking: lua scr.breakdown_level=df.interface_breakdown_types.QUIT
free(): invalid pointer

and on add-tests:

Tests completed in 17338 ms:
  374/377 tests passed
  5581/5583 checks passed
Invoking: lua scr.breakdown_level=df.interface_breakdown_types.QUIT
free(): invalid pointer

So the heap is already corrupted (or the teardown path frees a bad pointer) before DF exits; the test suite itself is green. Retries in the same attempt then fail earlier with Aborted (core dumped) right after dfhack: redirecting stderr to stderr.log, suggesting the first crash leaves the DF directory or process state in a way that breaks the follow-up launch.

This is separate from the Windows 60s-retry timeout that #5943 addresses, and it is currently masking otherwise-green runs.