Drive Joints opens on the picked joint; ui.start takes params (#1297)

Drive Joints opens on the joint picked in the browser, else the first joint that moves, and won't start on a rigid joint. ui.start takes params: joint.drive opens on a joint (id or name) at given values, driving nothing until OK; a command or param it doesn't take is refused.

docs/book/src/guide/assemblies.md

@@ -39,7 +39,8 @@ part still has with its loops closed: the moving parts of a four-bar have one be
 loop locked by a rigid joint has none. The Drive Joints dialog says when a drive stops short and
 why. While it previews, a row under the values gives the value the joint reached and the reason
 (the loop that can't close further, or the two parts a contact set keeps apart), and after OK the
-status line repeats it. The value box keeps what you typed.
+status line repeats it. The value box keeps what you typed. The dialog opens on the joint
+picked in the browser, or on the first joint that moves when none is picked.
 
 The solver places what it reaches from the root and from grounded components. A chain of joints
 that is joined to nothing grounded is never placed, so driving one of its joints moves nothing.

docs/control-protocol.md

@@ -105,7 +105,7 @@ unit regardless of the design, pass an expression with one: `"value": "50 mm"` o
 | `ui.set` | any `UiState` field (`tab`, `perspective`, `showGrid`, `dark`, …), and `hiddenBodies`, `hiddenOrigin`, `hiddenSketches`, `shownSketches`, `hiddenJoints` (whole lists; `browser.visibility` changes single items) | `dark: false` switches to the light theme |
 | `ui.view` | `view`: `front`, `back`, `top`, `bottom`, `left`, `right`, `iso`, `home`, `fit`; `animate` (bool, default false: snap) | |
 | `ui.camera` | `target?`: `[x, y, z]` (mm), `yaw?`, `pitch?`, `roll?` (radians), `distance?` (mm), `fov?`, `up?`; `fit?`: `true`, `false` or `[occurrence id or name…]` (`root`: the bodies outside every component) | the viewport's orbit camera: it looks at `target` from `distance` away; `yaw` 0 looks along +Y (from the front), `pitch` is the elevation (±π/2: straight down or up), `roll` turns the image. Fields given are set first (the others keep their values), then `fit` frames the listed occurrences (or everything), keeping the direction. The reply, passed back, sets the same view again, so a framing can be stored and replayed. `fov` and `up` follow the perspective setting (`ui.set`) and the up axis preference (`ui.prefs`) and can't change here; a refused field changes nothing |
-| `ui.start` | `command` | like a toolbar click: starts the sketch tool or dialog for the command |
+| `ui.start` | `command`, `params?` | like a toolbar click: starts the sketch tool or dialog for the command. `params` sets what the dialog opens on, and nothing runs until OK: `joint.drive` takes `joint` (id or name; without it, the joint picked in the browser, else the first joint that moves) and `values` (numbers in degrees or millimetres, or expressions). A command that takes no params is refused before it starts; a param the dialog doesn't take is refused and the dialog closes |
 | `ui.click` | `x`, `y` (screen points), `button?`: left/right/middle, `shift?`, `ctrl?`, `double?` | real pointer input |
 | `ui.move` | `x`, `y`, `shift?`, `alt?`, `ctrl?`, `cmd?` | pointer movement without a button; modifiers apply until the next synthetic move/click; a move with omitted modifiers releases them. With `nav: "touchpad"`, Shift pans and Alt/Option orbits. |
 | `ui.drag` | `x0`, `y0`, `x1`, `y1`, `button?`, `shift?`, `ctrl?`, `steps?`, `hold?`, `hold_end?` | press, move, release; a left drag on the model is a box selection (left to right: window, right to left: crossing); with OpenSCAD navigation, plain left-drag on empty space orbits, Shift+left-drag box-selects, and drags starting on sketch geometry edit it; `hold` keeps the button down that many frames first (a right press-and-hold opens the marking menu, releasing over a direction picks it), `hold_end` that many frames at the end (a sketch drag shows its snap) |