📦 Remove nlohmann_json from the public package contract

[burgholzer]

🤖 AI text below 🤖

Goal

Stop exposing and distributing nlohmann_json as part of MQT Core's public API and installed CMake package while retaining it privately for QDMI JSON parsing.

Current public exposure

• JSON-valued CoreDD statistics APIs.
• Neutral-atom configuration and FoMaC headers.
• An unnecessary public dependency of the SC configuration target.

Scope

1. Prepare QCEC, DDSIM, and QUSAT to own their JSON dependency or remove their JSON-facing APIs. QMAP already discovers nlohmann itself.
2. Remove nlohmann::json from public DD statistics; retain primitive counters and useful string/stream reporting.
3. Delete DDSIM's duplicate unused statistics serializer or make DDSIM own it.
4. Fold NA public-JSON cleanup into the QMAP transfer.
5. Make the SC configuration dependency private.
6. Remove Core's unconditional find_dependency(nlohmann_json), MQT_CORE_JSON_INSTALL, and custom header/CMake/natvis installation machinery.

Acceptance criteria

• No installed Core header exposes a nlohmann type.
• Installed shared and static Core targets contain no unresolved nlohmann target reference.
• QDMI JSON discovery and SC configuration retain robust private parsing.
• QCEC, DDSIM, QUSAT, QMAP, source installs, and wheels pass dependency inspection.