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/model/joint.ts

@@ -1,10 +1,10 @@
 /**
  * A joint ties the object it is placed below to another one (#154): a hinge turns round one point, a
  * spring pulls towards a length, a rope allows no more than one, and a fixed joint keeps the two as
- * they were placed. These are the keys of its rows; what the engine reads off them is in
- * `properties/joint.ts`.
+ * they were placed, and a slider lets it glide along one axis only. These are the keys of its rows;
+ * what the engine reads off them is in `properties/joint.ts`.
  */
-export const JOINT_TYPES = ['hinge', 'spring', 'rope', 'fixed'] as const;
+export const JOINT_TYPES = ['hinge', 'spring', 'rope', 'fixed', 'slider'] as const;
 export type JointType = (typeof JOINT_TYPES)[number];
 
 export const JOINT_TYPE_LABELS: Record<JointType, string> = {
@@ -12,6 +12,7 @@ export const JOINT_TYPE_LABELS: Record<JointType, string> = {
   spring: 'Spring',
   rope: 'Rope',
   fixed: 'Fixed',
+  slider: 'Slider',
 };
 
 export const JOINT_TYPE_KEY = 'jointType';
@@ -24,8 +25,28 @@ export const JOINT_DAMPING_KEY = 'damping';
 export const JOINT_LIMIT_KEY = 'limitAngle';
 export const JOINT_MIN_ANGLE_KEY = 'minAngle';
 export const JOINT_MAX_ANGLE_KEY = 'maxAngle';
+export const JOINT_LIMIT_DISTANCE_KEY = 'limitDistance';
+export const JOINT_MIN_DISTANCE_KEY = 'minDistance';
+export const JOINT_MAX_DISTANCE_KEY = 'maxDistance';
+export const JOINT_MOTOR_KEY = 'motor';
+export const JOINT_MOTOR_SPEED_KEY = 'motorSpeed';
+export const JOINT_MOTOR_TARGET_KEY = 'motorTarget';
+export const JOINT_MAX_FORCE_KEY = 'maxForce';
 export const JOINT_COLLIDE_KEY = 'collideConnected';
 
+/** What a motor on a hinge or a slider does: nothing, turn or glide on, or go to a place and stay. */
+export const JOINT_MOTORS = ['off', 'speed', 'target'] as const;
+export type JointMotor = (typeof JOINT_MOTORS)[number];
+
+export const JOINT_MOTOR_LABELS: Record<JointMotor, string> = {
+  off: 'Off',
+  speed: 'Speed',
+  target: 'Target',
+};
+
+/** The types that move along one axis, which a motor can drive and a position can be read of. */
+export const AXIS_TYPES: readonly JointType[] = ['hinge', 'slider'];
+
 /** The types with a point of their own on the other end; a hinge and a fixed joint have the one they sit on. */
 export const TWO_ANCHOR_TYPES: readonly JointType[] = ['spring', 'rope'];