Add downstream Julia CI using locally-built libdaqp

[Copilot]

No CI existed to catch regressions in the C library that would break downstream Julia packages (ParametricDAQP.jl, ASCertain.jl, PolyDAQP.jl, LinearMPC.jl).

Changes

• interfaces/daqp-julia/src/DAQPBase.jl — At module-load time, check for a libdaqp.{so,dylib,dll} next to the package root and, if found, override DAQP_jll.libdaqp to point at it. This is the same pattern already in core_tests.jl, lifted to module scope so any downstream package that Pkg.develops this DAQPBase inherits the override automatically — no changes required in downstream test files.

  let _local_lib = joinpath(@__DIR__, "..", "libdaqp." * Libc.Libdl.dlext)
      if isfile(_local_lib)
          @info "DAQPBase: using local libdaqp at $_local_lib"
          DAQP_jll.libdaqp = _local_lib
      end
  end

• .github/workflows/ci_downstream_julia.yml — New workflow (push/PR to master) that:

  1. Builds libdaqp.so via CMake with -DJULIA=ON; the sync_julia_interface target copies the library alongside the Julia sources in build/interfaces/daqp-julia/.
  2. Runs four parallel matrix jobs (fail-fast: false), one per downstream package — clones the repo, Pkg.develops the local DAQPBase, and calls Pkg.test(). The local library is picked up automatically via the DAQPBase change above.

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