DOCFIX-181: fix carve_gua subnet-enumeration hang (gauntlet was stalling) ...

netbox/dc-dc-prefixes-import.py::carve_gua() list()-ed every /64 subnet of the
GUA prefix (2**24 = ~16.7M for D-101's /40 example) only to use the first ->
hung/OOM. Real import runs would hang identically, not just the test. Fixed to
take the first /64 lazily via next() (O(1)).

Added a faulthandler watchdog to tests/dc-dc-prefixes-import/test_logic.py so a
future blocking regression self-aborts with a traceback after 30s instead of
silently stalling the whole run-tests-all.sh gauntlet -- the false-green that
let the ledger claim "40/40" for a test that never actually completed.

tests/dc-dc-prefixes-import: 40/40 in 0.05s (was: hung indefinitely).
Full gauntlet: ALL GREEN (52 harnesses). repo-lint 0 fail.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>