Various fixes and improvements across the whole project

[leducp]

Data loss & crashes:

• serialize: parse labels before links so label-routed wires survive save/load
• engine: fail build with MissingInput/DuplicateInputWiring instead of throwing at tick
• app: wrap engine play()/tick() so a compute exception stops the engine, not the editor
• migrate: refuse to overwrite input, reject already-migrated and empty-result files

Core robustness:

• contain malformed JSON fields as per-item diagnostics instead of leaking json::type_error
• reject id 0 / negative ids; guard reserve_ids_above overflow
• throw on non-array "pipelines"; validate/clamp theme grid spacing and font size
• fix AddStage/AddModeProfile undo deleting pre-existing entries
• repair_label_clusters honors first-by-id; registry heterogeneous lookup; add Lint kind

Memory safety (Python bindings):

• find_node/find_link/find_attr return copies; engine handles are shared_ptr surviving build()
• Step accessors throw instead of segfaulting; Stage keeps name alive; complete Diagnostic.Kind
• guard external-IO downcast (FactoryTypeMismatch); delete Engine copy/move; probe sink steps

Performance:

• cache node geometry per frame (kills O(pins^2) CalcTextSize); cull off-screen links
• debounce live-engine rebuild; pre-resolve per-stage blocks; drop per-frame exception throws

Build/packaging/docs/CI:

• fix README quick start; declare conan dep; fatal setup failure
• install rules + cmake.install packaging + version; gate IPO out of package builds
• BUILD_TESTS without BUILD_APP; pin GLFW commit; MIT license notices
• add GitHub Actions CI running the full suite