💥 Depend on nlohmann_json privately

[denialhaag]

Description

🤖 AI text below 🤖

MQT Core no longer exposes nlohmann_json as part of its public package contract. It uses the library only inside its implementation, for QDMI device discovery and superconducting device configuration, and stops installing it, exporting it, and looking for it in its package configuration.

The decision-diagram statistics drove most of the public exposure. They now report through strings and streams: json() on Statistics, MemoryManagerStatistics, TableStatistics, and UniqueTableStatistics, as well as UniqueTable::getStatsJson, getStatistics, and getDataStructureStatistics, are removed. The primitive counters, toString(), and the stream operator remain, and getStatisticsString and the new getDataStructureStatisticsString return the same report as a JSON-formatted string. Rendering moved to a header that is not installed, so the reports keep their format while no installed header includes a nlohmann header.

The remaining users link the library through $<BUILD_INTERFACE:>. Without the generator expression, a private dependency of a static library still appears as a $<LINK_ONLY:> entry in the installed interface, which would leave consumers resolving a target that MQT Core no longer provides.

This also removes the MQT_CORE_JSON_INSTALL option together with the machinery that installed the library's headers, package configuration, and natvis file.

Consumers that use nlohmann_json themselves have to depend on it directly. MQT QCEC, MQT DDSIM, and MQT QuSAT already do; MQT QMAP always did.

This pull request forms a stack with #2137 and targets that pull request's branch rather than main.

Fixes #2099

AI notice

This PR and its contents were created with the assistance of Opus 5 via Claude Code.

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
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[codecov]

Codecov Report

❌ Patch coverage is 93.84615% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/dd/statistics/MemoryManagerStatistics.cpp	88.2%	2 Missing ⚠️
src/dd/statistics/Statistics.cpp	0.0%	1 Missing ⚠️
src/dd/statistics/TableStatistics.cpp	93.3%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[burgholzer]

Very clean. Just one minor suggestion 😌