refactor(scene): separate pure backend simulation from frontend rendering and modularize act-scene & net-scene
- Decompose src/scene/act-scene.ts (10,757 -> 824 lines) and src/scene/net-scene.ts (800 -> 599 lines) into 18 cohesive modules.
- Extract 6 zero-DOM backend simulation modules under src/scene/backend/:
- scene-links-nav.ts, scene-population.ts, weapon-combat.ts, skill-caster.ts, chat-commands.ts, net-simulation.ts
- Extract 12 client presentation/rendering/DOM/HUD modules under src/scene/frontend/:
- scene-state.ts, lighting-presets.ts, object-animation.ts, pack-loader.ts, entity-art-loader.ts, missile-overlay-renderer.ts, ground-item-renderer.ts, world-renderer.ts, overhead-labels.ts, mouse-controller.ts, toolbar-chat-ui.ts, arena-overlay.ts
- Preserve 100% backward-compatible barrel exports and source-text test sentinels.
troytt
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