fix(bindings): mp complex → real numpy cast discards imag, not SIGABRT (3.3.1.dev1)

[ofloveandhate]

What

Fixes a hard interpreter crash (SIGABRT) when a multiprecision complex value with a
nonzero imaginary part is stored into a real (float64) numpy array — and ships it as
3.3.1.dev1.

M = np.zeros((r, c))                       # float64
M[i, j] = solver.real_solutions()[k][0]    # complex_mp w/ ~1e-13 imag  → SIGABRT (pre-fix)

Why it crashed

The registered complex_mp → double numpy cast did static_cast<To>(from), which routes
through boost.multiprecision's complex→scalar conversion. That throws
std::runtime_error("Could not convert imaginary number to scalar.") for any nonzero
imaginary part. numpy invokes the cast from an internal C loop that is not exception-safe, so
the C++ throw unwinds to an implicitly-noexcept boundary → std::terminate() → SIGABRT,
instead of a catchable Python error. Solution coordinates always carry tiny (~1e-13)
imaginary noise and eval returns a complex_mp, so this is trivially hit (it surfaced
building a Macaulay matrix for a local-dimension test).

Fix

The complex_mp → real cast now converts the real component, mirroring numpy's builtin
complex128 → float64 cast (which discards the imaginary part). One line in
cast<complex_mp, To> (python_bindings/include/eigenpy_interaction.hpp). This is the only
real-target cast registered for complex_mp — → complex128 has its own specialization
(preserves both parts) and → integer is unregistered — so nothing else moves. ADR-0055
records the guardrail (don't restore the static_cast form).

Tests

The eigenpy_numpy numpy-interop suite was never collected by pytest (its filename
matched neither test_*.py nor *_test.py) — renamed to eigenpy_numpy_test.py so its 15
existing tests actually run in CI, plus 7 new regression tests for the mp → narrower-dtype
casts: scalar setitem (the exact crash), vectorized .astype, broadcast assignment, parity
with the numpy builtin complex128 → float64, and a complex_mp → complex128 no-regression
check. Full python/test/classes suite green (571 passed).

Release

Second commit is the prep: CHANGELOG [3.3.1] + VERSION → 3.3.1.dev1. Once merged I'll tag
v3.3.1.dev1 (→ TestPyPI); after that's green, v3.3.1 for real.

🤖 Generated with Claude Code