feat(core): add AABB collision and rigid-body physics stepping

core/include/vf/Physics.h

@@ -0,0 +1,51 @@
+#pragma once
+
+#include "World.h"
+#include "Math.h"
+
+namespace vf {
+
+struct Aabb {
+    Vec3 min;
+    Vec3 max;
+};
+
+struct Player {
+    Vec3 position;   // feet centre (bottom of the collision box)
+    Vec3 velocity;
+    float yaw = 0.0f;
+    float pitch = 0.0f;
+    float width = 0.6f;
+    float height = 1.8f;
+    float eyeHeight = 1.62f;
+    bool onGround = false;
+    bool flying = false;
+};
+
+struct MoveInput {
+    float forward = 0.0f;   // -1..1
+    float strafe = 0.0f;    // -1..1
+    bool jump = false;
+    bool up = false;        // fly ascend
+    bool down = false;      // fly descend
+};
+
+struct PhysicsConfig {
+    float walkSpeed = 4.6f;
+    float flySpeed = 9.0f;
+    float gravity = 26.0f;
+    float jumpSpeed = 8.4f;
+    float maxFallSpeed = 55.0f;
+};
+
+Aabb playerBox(const Player& player);
+Vec3 eyePosition(const Player& player);
+
+bool boxOverlapsSolid(const World& world, const Aabb& box);
+
+// Advances the player one fixed step: applies input, gravity/fly, then resolves
+// AABB collisions against solid voxels axis by axis (move and slide).
+void stepPlayer(const World& world, Player& player, const MoveInput& input,
+                const PhysicsConfig& config, float dt);
+
+} // namespace vf