Add M2 collision mesh parsing and mesh-based wall/floor collision

Parse bounding vertices, triangles, and normals from M2 files and use
them for proper triangle-level collision instead of AABB heuristics.
Spatial grid bucketing for efficient queries, closest-point wall push
with soft clamping, and ray-triangle floor detection alongside existing
AABB fallback.

include/pipeline/m2_loader.hpp

@@ -200,6 +200,11 @@ struct M2Model {
     // Particle emitters
     std::vector<M2ParticleEmitter> particleEmitters;
 
+    // Collision mesh (simplified geometry for physics)
+    std::vector<glm::vec3> collisionVertices;
+    std::vector<uint16_t> collisionIndices;      // 3 per triangle
+    std::vector<glm::vec4> collisionNormals;     // xyz=normal, w=distance; one per triangle
+
     // Flags
     uint32_t globalFlags;