fix: propagate a raising key __eq__ instead of masking it as SystemError (#36)

[toloco]

Closes #36

What & why

CacheKey::eq and BorrowedArgs::equivalent compared PyObject_RichCompareBool(...) == 1. RichCompareBool returns -1 with a Python exception set when a key's __eq__ raises — and == 1 maps that to "not equal", leaving the exception pending on the thread. The lookup reports a spurious miss; the call path recomputes / returns Ok while an exception is set; PyO3 then raises a confusing SystemError: ... returned a result with an exception set, masking the user's real error (e.g. RuntimeError). functools.lru_cache checks the -1 sentinel; we didn't.

Reachable purely from safe Python: cache a key whose __hash__ collides with a stored key and whose __eq__ raises (e.g. f(Bad(42)); f(Bad(42))).

The fix

Both comparisons now go through a shared key.rs::rich_compare_eq helper:

• returns false ("not equal") for both 0 and -1, but on -1 leaves the exception set for the caller to fetch;
• bails early (returns false) if an exception is already pending, so a multi-key probe never re-enters the interpreter with a live exception and clobbers the original error.

Each memory-backend lookup site then calls PyErr::take(py) after the lookup and returns the error instead of proceeding:

• __call__ — read path (before recomputing) and the write-path double-check (before inserting),
• get, _probe.

evict_one and the write-path remove/insert compare a key against itself (same PyObject*), so RichCompareBool short-circuits to 1 via the identity fast path and never runs __eq__ — no check needed there.

Test (tests/test_raising_eq.py)

A key whose __eq__ raises now propagates the original exception through __call__, get, and _probe; the cache stays usable afterward; and hash-colliding keys that compare cleanly still cache independently (guards against over-eager error detection).

Verified fail-before → pass-after by stashing just the fix: the three read-path tests + the resilience test raised SystemError: ... returned a result with an exception set; with the fix all five pass.

Gates run (risky change — src/key.rs key equality / FFI boundary)

• make fmt / make lint (ruff, ty, clippy -D warnings) ✓
• make test — cargo test (11) + pytest (98, +5 new) ✓
• make test-matrix — Python 3.10–3.13 ✓, new tests pass on each (3.14 skipped locally via the documented uv-resolves-stale-alpha guard; CI covers 3.14 final)
• make bench — no regression: the added per-comparison PyErr_Occurred() is a TLS read; memory backend ~18.7M ops/s, single-thread 13.6–22.2M, within run-to-run variance

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