The backend listed every pair of shapes that may collide as an explicit
<pair>, with native filtering switched off. Model size, compile time and
per-step collision work grew with the square of the shape count: a chess
set of ~700 box colliders never finished compiling, and 600 boxes ran out
of memory.

Each geometry now carries contype/conaffinity bits and its own contact
parameters. Collision groups collide only when each admits the other,
MuJoCo's bitmasks when either does, so each distinct group gets fresh
bits that collide exactly as the groups do. Bodies joined without
collideConnected, and kinematic bodies against non-dynamic ones, become
<exclude> body pairs. Friction combines as MuJoCo's maximum instead of the
geometric mean. A FixedJoint with collideConnected is an error, since its
bodies compile into one MuJoCo body.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>