refactor: extract lab.html sim initializers to lab-init.js
lab.html: 9273 → 5180 lines (-44%) frontend/js/labs/lab-init.js: new file, 4098 lines All 34 _open*() functions + sim instance vars + openSim/closeSim/ _addTouchSupport/_simShow + THEORY data extracted to lab-init.js. Shared globals (FN_COLORS, ALL_SIM_BODIES, ALL_CTRL_BARS, sim vars) converted from const/let to var for cross-script accessibility. Three vars in lab.html made global: _disabledSimIds, _theoryOpen, _embedMode. Load order preserved: lab-init.js added before main <script> block.
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