refactor: remove dead icon_registry, orphan dirs; fix CMake drift; decompose node_graph_widget; exe-relative install data roots

[striderZA]

Summary

Maintainability cleanup from a comprehensive codebase review (findings M1, M2, M3, M4, M8, M9).

• M1 — delete dead icon_registry module. IconRegistry had zero callers (widgets draw schematic symbols via drawSchematicSymbol); removed the module, its member/accessor in NodeGraphWidget, and the stb/OpenGL texture plumbing that was linked PUBLIC into every build.
• M2 — remove orphaned dirs. extensions/device-generator (untracked binary, no manifest, never discovered) and rf-sim-generator-input/ (empty untracked scratch).
• M4 — drop spurious engine→UI CMake link. pfb_channelizer_engine is pure DSP; removed the simulator::core PUBLIC link that dragged imgui/glfw/OpenGL into every consumer including pure-DSP tests.
• M8 — decompose node_graph_widget.cpp (1152 → 542 lines). Extracted schematic symbols, group interaction, and tooltips into three TUs (pure move, byte-verified; public API unchanged).
• M9 — CMake hygiene. Added missing project()/cmake_minimum_required to attenuator/logging/common; fixed attenuator's bare include path; factored tests/CMakeLists' 9 repeated standalone-test blocks into add_standalone_test(...) with a shared engine-lib variable.
• M3 — install story. Library/extension scan roots now resolve exe-relative (<exe_dir>/component_data/library, <exe_dir>/extensions) with a source-tree fallback for dev builds; install rules ship component_data/ and extensions/ next to the exe. Installed binaries previously shipped with an empty library and no extension support.

Verification

• Full build green; tests.exe 216 cases / 65530 assertions; test_ui 27/27 — all baseline-identical (pure refactor, no test additions).
• Staged-install check: bin/ + bin/component_data/library/** + bin/extensions/ land as the exe-relative lookups expect; app launches cleanly.
• ctest -N total unchanged.

Notes for merging

• tests/CMakeLists.txt is intentionally not part of this PR — the add_standalone_test refactor and the new containment-test registration travel in fix/crash-and-containment (which owns that file).
• The component_engine_base.h additions land in fix/dsp-accuracy and refactor/engine-unification, not here.