feat: enhance scene graph with entity system improvements

[mtao]

Summary

Adds four incremental enhancements to the visualization scene graph's Object+AbstractFeature pattern, improving query performance, enabling reactive updates, and providing convenient traversal utilities.

Changes

1. O(1 feature lookup — Object now maintains an unordered_map<type_index, AbstractFeature*> alongside the existing vector<unique_ptr<AbstractFeature>>. find_feature<F>() checks the map first (exact type match), falling back to dynamic_cast scan for base-class queries (only 4 of 55 existing call sites need this).

2. Feature signals — Adds palacaze/sigslot v1.2.3 as a dependency. New signals:

   • Object::on_feature_added(Object&, AbstractFeature&)
   • Object::on_feature_removing(Object&, AbstractFeature&)
   • Object::on_child_added(Object&, Object&)
   • Object::on_child_removing(Object&, Object&)
   • AbstractFeature::on_modified(AbstractFeature&) with mark_modified() helper

3. Scene-level FeatureIndex — Subscribes to Object signals and maintains a type_index -> vector<Object*> index. Provides objects_with<F>() (returns span) and count<F>() for O(1) "give me all objects with MeshData" queries. Auto-tracks/untracks children as the scene graph changes.

4. Traversal API — New methods on Object:

   • for_each_descendant(fn) — DFS pre-order; if fn returns bool, false prunes subtree
   • for_each_ancestor(fn) — walks parent chain; false stops early
   • find_descendant(pred) — returns first matching Object* or nullptr

Testing

• 25 new test cases added to test_scene_graph.cpp covering all four enhancements
• All 8 test suites pass: 23,717 total assertions

Design Notes

• This enhances the existing Object+Feature pattern for the visualization layer (10-100 objects). A separate full ECS (e.g., entt/flecs) would be used in archer for simulation-scale data (10k-1M entities) if needed.
• sigslot::signal_st is used for zero-overhead single-threaded operation. observer_st base class provides automatic disconnect on destruction.