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>

src/core/graph/nodes/action/physicsNodes.ts

@@ -95,6 +95,24 @@ export const GetVelocity: NodeDef = {
   },
 };
 
+export const JointPosition: NodeDef = {
+  type: 'JointPosition',
+  label: 'Joint position',
+  category: 'data',
+  description:
+    'Where a hinge or a slider stands while the game is played: how far it has turned (degrees, ' +
+    'a wheel counts on past a full round) or glided (world units) from how it was placed, and how fast ' +
+    'that changes. 0 for a spring, a rope, a fixed joint, a joint that holds nothing, and in the editor run.',
+  inputs: [obj('joint')],
+  outputs: [num('value'), num('speed')],
+  params: [],
+  run: (ctx, inputs) => {
+    const joint = asEntity(inputs.joint);
+    const state = (joint && ctx.physics?.jointStateOf(joint)) || { value: 0, speed: 0 };
+    return { outputs: { value: state.value, speed: state.speed } };
+  },
+};
+
 export const SetGravity: NodeDef = {
   type: 'SetGravity',
   label: 'Set gravity',

src/core/graph/nodes/index.ts

@@ -12,6 +12,7 @@ import {
   ApplyForce,
   ApplyImpulse,
   GetVelocity,
+  JointPosition,
   SetAngularVelocity,
   SetGravity,
   SetVelocity,
@@ -277,6 +278,7 @@ export const BUILT_IN_NODES: readonly NodeDef[] = [
   Spawn,
   Destroy,
   AddJoint,
+  JointPosition,
   Emit,
   PlayAnimation,
   PauseAnimation,