Add C API constructors for classical expressions

[DennisWayo]

Fixes #16667.

Summary

• add public C API constructors for standalone classical-expression variables, stretches, values, and expression nodes
• add ownership/free functions for created QkVar, QkStretch, QkValue, and QkExprNode objects
• register the new exports in the C API ABI slot table and vtable
• add C tests that build expression trees with the new public API and inspect the resulting metadata

This ports the useful API shape from the older WIP #16180 onto current main, while leaving big-integer value construction for follow-up after #16666 lands.

Verification

• cargo check -p qiskit-cext
• make ctest
• cargo run --quiet -p qiskit-bindgen-cli -- check-abi capi_slots.txt
• cargo check -p qiskit-cext-vtable --features addr
• cargo fmt --check --package qiskit-cext --package qiskit-cext-vtable
• git diff --check

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[jakelishman]

I believe this, and the 4 others opened concurrently, to be an entirely LLM-generated PR with little human oversight, which is against the contribution guide. Given past experience, it is unlikely to be worth maintainer time to review.

If you wish to meaningfully contribute, slow down, keep one or two PRs open max, and engage as a human, even if you use LLM assistance.