Joints: gears, rack and pinion, and screws (#1524)

Motion links can carry a coupling (gear pair, rack and pinion, screw) whose ratio follows radii, picked round faces or pitch. Design format 23.

Co-authored-by: SychPL <SychPL@users.noreply.github.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/book/src/guide/assemblies.md

@@ -28,6 +28,33 @@ made. Contact sets stop motion where bodies would pass through each other,
 **Motion Study** keys joint values along a timeline, and **Interference** shows overlapping
 volumes in red.
 
+**Gear**, **Rack and Pinion** and **Screw** (on the **ASSEMBLE** panel) are motion links whose
+ratio comes from the parts rather than a typed number:
+
+- **Gear** (`joint.gear {a, b, ra, rb}`) couples two joints that turn. Joint 2 turns ra/rb as far
+  as joint 1, the other way round; **Internal gear** (one of them is a ring gear) turns both the
+  same way. Pick the round face of each gear, such as its pitch cylinder, or type the radii. Tooth
+  counts work as well as radii, since only their proportion counts. Picking a gear's face also
+  chooses the joint that gear turns on.
+- **Rack and Pinion** (`joint.rack_pinion {a, b, radius}`) couples a joint that turns with one
+  that slides: the rack travels the pinion's pitch radius times its turn in radians (r·θ). Either
+  joint may drive the other. In `joint.list` the ratio is given per degree, as for any link between
+  an angle and a distance.
+- **Screw** (`joint.screw {joint, pitch}`, or `{a, b, pitch}` for a revolute joint and a slider)
+  advances `pitch` per turn. A cylindrical joint is a screw on its own: its turn drives its travel.
+  The thread is right-handed; **Reverse** makes it left-handed.
+
+A radius or pitch may be an expression of the design's parameters, and a picked face is found
+again whenever the design changes. Change the parameter or the gear's diameter, and the ratio
+follows. The new link leaves the driven joint where it is, so teeth modelled in mesh stay in mesh.
+**Reverse** turns the driven joint the other way, for joints whose axes point opposite ways. These
+couplings are motion links, so they work in closed loops, with contact sets and in motion studies:
+a rack can push the piston of a slider-crank, or a pinion can steer through a rack and its tie
+rods. Drive the joint that drives the coupling (joint 1, the pinion, or the screw's turn), not the
+driven one. A joint follows one link at most, and a coupling that would drive its own driver is
+refused. `joint.list` lists each coupling's definition under `links` with an `error` when it can't
+be worked out (the picked face is gone); the link then keeps its last ratio.
+
 A joint moves one component against another, so its two picks must be on different components.
 A STEP file with several parts often comes in as bodies of one component, and picks on two of
 those bodies make no joint. The Joint and As-built Joint dialogs say so under the picks and offer

docs/design-container.md

@@ -7,7 +7,7 @@ self-contained input/output format. `io::write_design` keeps its JSON contract;
 `io::write_design_container` is the fallible container writer.
 
 Packages have `container.json` with `{"format":"solidcraft-container/1"}` and
-`document.json` with the versioned parametric document (currently `solidcraft/22`).
+`document.json` with the versioned parametric document (currently `solidcraft/23`).
 The independent versions let document migrations evolve without changing blob encodings.
 Existing serialized document fields are retained automatically, without a second copy
 of the document schema or a deep clone of embedded payloads.

docs/features.md

@@ -1,6 +1,6 @@
 # Features
 
-SolidCraft's commands by area, generated by `cargo xtask features` from the command registry (`solidcraft-cli commands`). **373 commands.** Planned (registered, not built yet): 3. Every command runs the same way from the toolbar, a script, `solidcraft-cli exec <id> '<json>'`, the control channel and MCP; parameters are in the [command reference](book/src/reference/commands.md) (`cargo xtask book`). Status and plans: [ROADMAP](../ROADMAP.md).
+SolidCraft's commands by area, generated by `cargo xtask features` from the command registry (`solidcraft-cli commands`). **376 commands.** Planned (registered, not built yet): 3. Every command runs the same way from the toolbar, a script, `solidcraft-cli exec <id> '<json>'`, the control channel and MCP; parameters are in the [command reference](book/src/reference/commands.md) (`cargo xtask book`). Status and plans: [ROADMAP](../ROADMAP.md).
 
 ## Sketch (102)
 
@@ -246,7 +246,7 @@ SolidCraft's commands by area, generated by `cargo xtask features` from the comm
 | Break Link | `component.break_link` | – |
 | List Links | `component.links` | – |
 
-## Assembly joints (11)
+## Assembly joints (14)
 
 | Command | Id | Toolbar |
 |---|---|---|
@@ -256,6 +256,9 @@ SolidCraft's commands by area, generated by `cargo xtask features` from the comm
 | Rigid Group | `joint.rigid_group` | SOLID › ASSEMBLE |
 | Drive Joints | `joint.drive` | SOLID › ASSEMBLE |
 | Motion Link | `joint.motion_link` | SOLID › ASSEMBLE |
+| Gear | `joint.gear` | SOLID › ASSEMBLE |
+| Rack and Pinion | `joint.rack_pinion` | SOLID › ASSEMBLE |
+| Screw | `joint.screw` | SOLID › ASSEMBLE |
 | Joint Limits | `joint.limits` | – |
 | Edit Joint | `joint.edit` | – |
 | Delete Joint | `joint.delete` | – |