Qiskit / qiskit-fermions / 34473493707

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Commit Message

Fill gaps in the C API (#340)

* docs(cext): document the operand order of qf_*_compose

The four `qf_*_compose` docstrings described their parameters only as `left`
and `right` and their result as "the resulting operator", which left a C caller
unable to tell whether they receive `A B` or `B A`. The distinction is real
rather than cosmetic: `composed` and `matmul` are separate methods in the core
crate, and `qf_*_compose` wraps only the former.

Qiskit's C API resolves the same ambiguity for `qk_obs_compose` in prose rather
than by shipping a second symbol, so follow that precedent and spell the
convention out, including the note that the other order is obtained by swapping
the arguments. That sentence is what makes the absence of a `qf_*_matmul`
defensible, and it is exact: core's `matmul` is literally `_compose` with its
arguments swapped.

Also document that the composition drops group indices, which the core
implementation does unconditionally but no C docstring mentioned.

The accompanying test pins the documented order so that it cannot drift
silently. It deliberately uses a non-commuting pair, since the `one`/`zero`
operands used by the existing `test_compose` satisfy both conventions and would
therefore assert nothing about the operand order.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* feat(cext): expose the operator support in the C API

`get_support` was reachable from Python but had no C counterpart at all, even
though the underlying `OperatorTrait::get_support` is implemented for every
operator representation in the core crate.

The other getters hand out a borrowed pointer into the operator, which is not an
option here: the support is gathered into a set on demand rather than stored, so
there is no buffer to lend out. Use the caller-sized output buffer pattern
already established by `split_out_groups` and the group-analysis functions, which
means the support needs a companion `num_support` to size that buffer with.

The ind... (continued)

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