Generator refactor groundwork: golden tests + indentation context manager (PoC)

[devdanzin]

Summary

Groundwork for migrating the code generators off the manual self.write()/addLevel()/restoreLevel() indentation idiom (the systemic complexity driver in doc/code-review-findings.md).

Golden-output harness (tests/python/test_golden_output.py) — WritePythonCode is a pure function of (RNG seed, options, target module), so we pin all three and snapshot the generated source.py. This is the regression net that lets generator refactors be proven byte-for-byte behaviour-preserving. Determinism is engineered: fixed seed; fully-pinned options (no MagicMock id leakage); a fake module with a public-only surface (test_private=False makes the generator skip dunders → version-stable introspection); no_numpy/no_tstrings → output independent of installed deps. Three checks: determinism+parse (all versions), the converted method's block (version-independent), and a full-script snapshot (pinned to the canonical >=3.14). The golden/ dir is ruff-excluded (generated fuzzer output, kept byte-exact).

Indentation context manager (fusil/write_code.py) — with self.indented(): replaces the saved = addLevel(1); …; restoreLevel(saved) (and error-prone restoreLevel(self.base_level - 1)) bookkeeping with a scoped block whose nesting mirrors the generated code and can't leak a level. Named indented (not indent) because self.indent is the indent string. Covered by tests/test_write_code.py.

Proof of concept — the class-instantiation try/except/if triple-dance in write_python_code.py now uses with self.indented():; the golden snapshot proves the generated output is byte-identical.

Test plan

• Full suite: 320 → 328 OK (+8: 5 write_code, 3 golden).
• Golden snapshot byte-identical before and after the PoC conversion.
• ruff check + ruff format --check clean (golden fixture excluded).

This unblocks the incremental generator cleanup: convert methods to with self.indented(): (and templates for fixed-shape blocks) one at a time, with the golden net catching any drift.

Closes #143