Add Box3D physics for dynamic objects - #1307

Simulates prop_physics / prop_physics_multiplayer / prop_physics_override
through Box3D.NET: world_physics is pushed as a static compound shape,
props become dynamic bodies built from their spheres, capsules and hulls,
and the entity tick steps the world at 64 Hz.

The player shoves props through a kinematic hull body (soft collision
only, props never block movement), and E picks up the prop under the
crosshair: rigid velocity tracking holds it centered on the mass center
in front of the eyes, glued to the view per frame while unblocked, with
a smooth attach glide and full velocity inheritance on release.

Friction, elasticity, density and impact sounds are data driven from
surfaceproperties.vsurf, matched hash-to-hash with each shape's surface
property and played off Box3D contact hit events.