Summary

• Add a BinaryBuilder recipe for MQLib_jll that keeps the existing MQLib executable product and adds LibraryProduct("libmqlib_c_api", :libmqlib_c_api).
• Build libmqlib_c_api from the upstream MQLib sources plus the C ABI wrapper, excluding src/main.cpp, with Windows export support via MQLIB_C_BUILD_SHARED.
• Document the JLL library product name, ABI version expectations, and downstream ccall entry point.
• Extend the C ABI contract tests to verify the recipe keeps the executable product and declares the shared library product.

Tests run

• julia +1.12 --project=. -e 'import Pkg; Pkg.test()' - passed: Compatibility metadata 5/5, QUBODrivers 124/124, and C ABI contract 16/16.
• julia +1.10 --startup-file=no -e 'import Pkg; Pkg.activate(; temp=true); Pkg.develop(path=pwd()); Pkg.test("MQLib")' - passed: Compatibility metadata 5/5, QUBODrivers 124/124, and C ABI contract 16/16.
• bash test/native_c_api_smoke.sh - passed; cloned upstream MQLib, built libmqlib_c_api.so, and ran the native solve smoke cases.
• julia +release --project=. -e 'Meta.parseall(read("jll/build_tarballs.jl", String)); println("recipe parse ok")' - passed before the rebase.

Notes

• Direct julia +1.10 --project=. -e 'import Pkg; Pkg.test()' did not run locally because the checked-in Manifest.toml was generated by Julia 1.12 and Julia 1.10 could not instantiate its OpenSSL_jll entry. The Julia 1.10 package tests pass when resolved from Project.toml in a clean temporary environment.
• I did not run a full BinaryBuilder cross-platform tarball build locally.

Refs #7