generated code: keep source.py parseable/runnable on a pre-3.13 target (PyPy 3.11)

[devdanzin]

Why

fusil's requires-python floor of 3.13 constrains the runner. The generated
source.py is executed by --python, which is routinely older or alternative — PyPy 3.11
is a fuzzing target in its own right.

Two 3.12+/3.13+ constructs leaked into the emitted script and aborted every session
before any fuzzing happened:

1. f"{expr!r }" — a space between the conversion and the closing brace is PEP 701
   syntax (3.12+). On a 3.11 target the entire script is a SyntaxError, so nothing
   parses. Three emitter sites in write_python_code.py; tightening them to {expr!r}
   leaves the printed text byte-identical.
2. types.CapsuleType — 3.13+, read unguarded in the tricky-objects prelude. On PyPy
   3.11 every session died at that line with AttributeError. Now guarded exactly the way
   the neighbouring types.GenericAlias already is; the if tricky_capsule: use site
   immediately below already expects a falsy value.

Why this is worth a test

Both failure modes are invisible from the outside: the run looks clean — no crashes,
no kept dirs — because nothing ever ran. A 12-session PyPy run reported zero findings while
in fact zero calls had been fuzzed.

Measured on PyPy 3.11.15 / 7.3.23 (--discover-in-target, CPython 3.14 runner): sessions
went from dying in the prelude to thousands of lines of real activity each.

Tests

tests/python/test_target_py311_compat.py — verified each test fails when its fix is
reverted:

• execs the tricky-objects prelude under a types shim with CapsuleType removed, asserting
  it survives and yields None;
• scans every emitter under fusil/ for a conversion field followed by whitespace, and
  checks the golden snapshot for the same.

Full suite green (1247 tests); ruff check + ruff format --check clean. Golden snapshot
regenerated for the intentional output change.

🤖 Generated with Claude Code