--gc-aggressive killed every session on a PyPy target, silently

[devdanzin]

Found triaging ~/crashers/fusil-fleet-pypy_01, which reported 43 368 sessions, 0 crashes,
0 timeouts, 0 cpu-load kills. None of those sessions ran.

The bug

gc.set_threshold is CPython-only, and the prelude runs on the target. On PyPy the
unguarded call raises AttributeError at module level — line 55 of every generated script,
before a single target call:

  File "session-391/source.py", line 55, in <module>
    gc.set_threshold(1, 1, 1)
AttributeError: module 'gc' has no attribute 'set_threshold'

The failure mode is the dangerous part: the sessions don't look broken, they look clean.
Score 0.0%, duration 0.3 s, no crash, no timeout, nothing in any log saying why. A fleet can
run for hours reporting a tidy zero.

Verified on a real script from that fleet

	output	exit
as the fleet ran it	6 lines, AttributeError	1
with the guard	500 lines, --- Fuzzing 40 functions in traceback ---	1

The fix

import gc
try:
    gc.set_threshold(1, 1, 1)
except AttributeError:
    pass  # not CPython: no generational threshold to tighten

A no-op rather than an error on such a target: --gc-aggressive is a tuning knob, not a
request, and PyPy's GC is not generational in CPython's sense, so there is nothing to coerce.

Tests

Two, both resistant to the obvious ways this could regress:

• one walks the AST to assert the set_threshold call sits inside a try/except AttributeError, rather than matching source text that a reformat could break;
• one execs the emitted snippet against a gc module with no set_threshold — which is
  exactly what PyPy looks like from the prelude's point of view.

The recurring lesson

This is the same shape as the pre-3.13 target-compat break fixed in #256, where every session
also died in the prelude. The rule that keeps being relearned: anything CPython-specific in
the prelude must be guarded, because the prelude runs on the target. The faulthandler block
right above already does this and says so in its comment.

1286 tests OK, ruff check and ruff format --check clean.