Add flowchart manager for calculator example

[legend-df]

Summary

• introduce a FlowchartManager widget that allows multiple calculator flowcharts to be managed in a single window
• update the calculator example to use the manager and seed multiple tabs on startup
• register the new sources in the calculator CMake target so the new widget is built

Testing

• cmake -S . -B build (fails: missing Qt and unable to download Catch2 in the environment)

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https://chatgpt.com/codex/tasks/task_e_68eda61890e4832d99c8d679f7caa1b2

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid moc’ing FlowchartManager in headless target

The shared ${CALC_HEADER_FILES} list now contains FlowchartManager.hpp, which declares FlowchartManager and FlowchartPage with Q_OBJECT. Reusing that list for headless_calculator causes CMake’s automoc to generate meta-object code for those classes, but the target never builds FlowchartManager.cpp, so the moc object will reference undefined symbols such as FlowchartManager::addFlowchart() at link time. This breaks the headless example even though it never uses the flowchart manager. Consider either removing FlowchartManager.hpp from the header list or excluding it from the headless target to keep the build linking.

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