Let a joint glide on a rail and drive it with a motor (#154)

Step 2 of the joints: a door that opens by itself, a lift and a
piston need a joint that slides along one line and a motor that turns
or pushes. A graph also needs to read how far such a joint stands.

- Slider: a fifth joint type on Rapier's prismatic joint. Its rail is
  the joint's own x axis, so turning the joint turns the rail. Joints
  now carry Scale, Rotation and Mirror like a sprite.
- Limit distance with Min and Max, in world units from where the
  object was placed.
- Motor on hinge and slider: Off, Speed or Target, with Motor speed,
  Motor target and Max force (0 = no limit). Target goes there at no
  more than the speed and slows down for the last stretch.
- Changing speed, target or force keeps the engine joint. Switching
  the motor on or off builds it again.
- New node Joint position: value and speed of a hinge (degrees,
  counting past a full round) or a slider (world units).
- Scene view draws a slider's rail dashed, with a tick at each limit.
- shownFor in the schema takes a list of choices that must all hold.

Findings, measured with a probe against Rapier 0.20:
- A prismatic joint holds both bodies at one angle. Held to the scene
  or to an object without a rigidbody, the joint's own anchor body is
  turned by the placed difference, so nothing jumps. Between two
  rigidbodies the held one is turned to match, and the console says
  so once.
- Rapier reads no joint angle back. The position is worked out from
  the two bodies.
- A hinge motor is weak on a small body at the end of a long arm
  (box 20 on an arm of 100 does not lift itself). More solver
  iterations would help but cost every scene. A door of 100 x 20
  reaches -90 and holds it. This is written in the wiki.

Tests:
- Unit: 16 new in joints.test.ts, 1 in joint-links, 3 catalog cases.
- e2e 41: 27 checks (25 + 2), block 41f with a real mouse in the
  inspector and the motor in play (59.1 of 60 units in 1 s).
- 9 of 9 unit counter-probes fail as they should.
- Gate: 220 files, 3436 tests (3416 + 20), CHECK EXIT 0.
- e2e total 1701 is counted (1699 + 2), the full suite did not run.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core/node-catalog.test.ts

@@ -135,6 +135,7 @@ const physicsDouble = (): PhysicsDouble => {
     ),
     applyImpulse: (target, x, y) => (calls.push(['applyImpulse', target.id, x, y]), has(target)),
     setGravity: (x, y) => void calls.push(['setGravity', x, y]),
+    jointStateOf: (target) => (target.id === 'hinge' ? { value: 30, speed: -12 } : null),
   };
 };
 const physicsCalls = (ctx: FakeContext) => (ctx.physics as PhysicsDouble).calls;
@@ -1462,6 +1463,25 @@ const CASES: Record<string, Case[]> = {
     },
     { name: 'without physics it reads 0', inputs: { object: crate }, outputs: { x: 0, y: 0, angular: 0 } },
   ],
+  JointPosition: [
+    {
+      name: 'reads how far the joint stands from where it was placed, and how fast',
+      inputs: { joint: fakeEntity('hinge') },
+      context: { physics: physicsDouble() },
+      outputs: { value: 30, speed: -12 },
+    },
+    {
+      name: 'a joint the engine does not read reads 0',
+      inputs: { joint: fakeEntity('rope') },
+      context: { physics: physicsDouble() },
+      outputs: { value: 0, speed: 0 },
+    },
+    {
+      name: 'without physics it reads 0',
+      inputs: { joint: fakeEntity('hinge') },
+      outputs: { value: 0, speed: 0 },
+    },
+  ],
   SetGravity: [
     {
       name: 'sets the scene gravity',