Merge origin/main (6f52cc50, the squash of upstream pull request 471) into w/form-controls

Both sides added a kind of floating object (equation, form control); both are kept everywhere.
ROADMAP.md: header combined (form controls first, then equations), catalog 281 / 296 as `cargo xtask parity` measures, the form-controls progress-log and revision rows above main's equation rows.
crates/engine/src/cmd.rs: both spec lists (controls and equation).
crates/engine/src/cmd/extra.rs: the arrange params docs list both kinds.
crates/engine/src/cmd/extra/objects.rs: main's `found` match and `anchors_of` on ObjKind gain the Control arms; the unlocked check takes the parsed kind directly.
crates/engine/src/cmd/insert.rs: the object params docs and the kind error name both kinds.
crates/engine/src/tests.rs: both test modules.
crates/engine/src/tests/objects.rs: every_kind returns six objects (equation at L18, slider at P18) and the anchor lists carry both.
crates/model/src/features.rs: ObjKind has Equation and Control, ALL lists six kinds, name() both.
crates/model/src/sheet.rs: both imports, both accessor arms, the order test inserts equations (6, 8) and controls (13, 15) and documents controls after slicers with equations counted as shapes.
crates/model/src/workbook.rs: main's refresh_equations and this branch's free-id doc comment.
crates/ui-egui/src/chartview.rs: both doc comments combined.
crates/ui-egui/src/objects.rs: both OpenAction variants and arms.
crates/ui-egui/src/ribbon/insert.rs: the import list is the union.
docs/control-protocol.md: the kind list names equation and control; main's equation sentences and this branch's form-controls sentence.
docs/file-format-parity.md: header combined; the xlsx row carries both additions; the form-controls revision row above main's equation rows.
docs/parity-checklist.md: regenerated with `cargo xtask parity` (281 / 296).
docs/target-app-parity.md: header combined; catalog 281 / 296; the Insert objects row carries both additions; the form-controls revision row (277 / 292 → 281 / 296) above main's.
docs/ui-parity.md: header combined with catalog 281 / 296; main's move-and-resize row with equations plus the form-controls row; the form-controls revision row above main's.

docs/control-protocol.md

@@ -92,10 +92,14 @@ include `window` or `doc` to run it in that window, as with `engine.execute`; th
 them as their own parameters.
 
 `ui.inspect` reports the selected floating object as `selectedObject: {kind, id}` (or `null`),
-where `kind` is `chart`, `image`, `shape`, `slicer` or `control` (a form button, slider or spin
-button); the same names go in the `kind` parameter of `object.delete`, `object.move`,
-`object.setAnchorMode` and the `arrange.*` commands. Form controls are worked with
-`control.setValue` and `control.press`, and listed by `control.list`.
+where `kind` is `chart`, `image`, `shape`, `equation`, `slicer` or `control` (a form button, slider or spin
+button); the same names go in the `kind` parameter
+of `object.delete`, `object.move`, `object.setAnchorMode` and the `arrange.*` commands. An
+equation is a shape too, so its id works under `shape`; a plain shape's id given as `equation`,
+a `kind` that is present but not one of these names (`null` included), or an id of no object of
+the kind, is an error. `arrange.align` and `arrange.group` check every id in `ids` the same way;
+with fewer than two ids they change nothing.
+Form controls are worked with `control.setValue` and `control.press`, and listed by `control.list`.
 `object.move` and `chart.set` cap `width` and `height` at 10,000 points, or at the object's
 current size when a file made it larger: such an object keeps its size when it is moved and
 shrinks by what a resize asks, but does not grow further.