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fix(operators): stop conjugating coefficients in ichop (#253)

`ichop` pushed `term.coeff.conj()` rather than `term.coeff`, so chopping an
operator silently conjugated every *surviving* coefficient: an operator with a
`3+4j` term came back as `3-4j`. The `.conj()` was copy-pasted from the
neighbouring `adjoint`, where it is correct.

This is public API in both language bindings (`op.ichop(atol)` and
`qf_*_op_ichop`), and it affected all four operator types.

The one place `ichop` is used internally, `is_hermitian`, was unaffected: it
chops `self - self.adjoint()` and then compares against zero, and both the chop
filter and that comparison only read `abs()`, which conjugation preserves.
Verified over 300 random complex operators — no `is_hermitian` result changes,
so no regression test is added there (it would pass before and after).

Every pre-existing `ichop` test used purely real coefficients, where `.conj()`
is the identity, which is why all three test suites missed this. The new tests
therefore use genuinely complex coefficients, and keep a below-threshold term so
they also pin that the filter compares `abs()` and not the real part.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

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