stop scoring the fuzzer's own noise as target crashes

[devdanzin]

Two independent false-positive sources, both measured on a PyPy 3.11 stdlib fleet
(fusil-pypy311_fleet_02, 255 kept dirs). Together they account for 23 of 255 (9%) —
every one a session the fuzzer killed itself.

1. Three bomb signatures missing from the stdout ignore regex

The regex has been extended once per bomb family, but three raise sites were added later
without it:

signature	raised at	covered?
fusil instancecheck	bomb_objects.py:685 (metaclass bomb)	❌
fusil monitoring callback bomb	write_python_code.py:1807	❌
fusil junk return	write_python_code.py:1811	❌

instancecheck is raised as SystemError — a 1.0 word, so this isn't a cosmetic gap: it
manufactures crashes. 7 sessions kept in that one fleet.

test_ignore_regex_covers_every_raised_bomb_signature scrapes the raise sites and fails on
any signature the regex misses, so the next bomb family can't repeat this. Verified: with the
three alternatives removed it reports exactly those three.

2. default_int_handler kills the session

signal.default_int_handler / _signal.default_int_handler raise KeyboardInterrupt — a
BaseException, so it blows straight through the generated script's except Exception
handlers and takes the session with it. That's the #192 class.

Called directly as a fuzz target it tagged 16 dirs -sigint in this fleet, and the
rustpython fleets show the same pattern. It exists on every interpreter, so this is not
PyPy-specific. Its neighbours (pause/alarm/setitimer/pthread_kill) were already
blacklisted for exactly this reason — this one was just missed.

Full suite green (1254); ruff clean.

🤖 Generated with Claude Code