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.

crates/engine/src/cmd/insert.rs

@@ -146,13 +146,13 @@ pub fn specs() -> Vec<CommandSpec> {
             &with(p, "kind", json!("textBox"))
         )),
         cmd!("shape.setText", "Edit Text Box", [], None, "{id: textBox id, text: string}", has_doc, shape_set_text),
-        cmd!("object.delete", "Delete Object", [], None, "{kind: chart|image|shape|slicer|control, id}", has_doc, delete_object),
+        cmd!("object.delete", "Delete Object", [], None, "{kind: chart|image|shape|slicer|equation|control, id}", has_doc, delete_object),
         cmd!(
             "object.move",
             "Move Object",
             [],
             None,
-            "{kind: chart|image|shape|slicer|control, id, at?: \"C3\", dx?, dy?, width?, height? (each at most 10,000 points, or the object's current size when a file made it larger)}",
+            "{kind: chart|image|shape|slicer|equation|control, id, at?: \"C3\", dx?, dy?, width?, height? (each at most 10,000 points, or the object's current size when a file made it larger)}",
             has_doc,
             move_object
         ),
@@ -161,7 +161,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Object Properties",
             [],
             None,
-            "{kind: chart|image|shape|slicer|control, id, mode: moveAndSize|moveOnly|absolute}",
+            "{kind: chart|image|shape|slicer|equation|control, id, mode: moveAndSize|moveOnly|absolute}",
             has_doc,
             set_anchor_mode
         ),
@@ -1170,6 +1170,7 @@ fn insert_shape(s: &mut Session, p: &Json) -> Result<Json> {
         Color::Theme(4, -250)
     });
     let shape = Shape {
+        math: Default::default(),
         id,
         kind,
         anchor: Anchor {
@@ -1227,7 +1228,8 @@ fn shape_set_text(s: &mut Session, p: &Json) -> Result<Json> {
 /// objects are errors, so a stale id never succeeds silently.
 fn object_param(s: &Session, p: &Json, cmd: &str) -> Result<(usize, ObjRef)> {
     let id = u32_param(p, "id").ok_or_else(|| bad(cmd, "missing `id`"))?;
-    let kind = str_param(p, "kind").and_then(ObjKind::parse).ok_or_else(|| bad(cmd, "kind must be chart, image, shape, slicer or control"))?;
+    let kind =
+        str_param(p, "kind").and_then(ObjKind::parse).ok_or_else(|| bad(cmd, "kind must be chart, image, shape, slicer, equation or control"))?;
     let d = s.doc()?;
     let sheet = d.wb.active_sheet();
     let o = ObjRef::new(kind, id);
@@ -1786,6 +1788,7 @@ fn insert_icon(s: &mut Session, p: &Json) -> Result<Json> {
     let at = cell_param(s, p, "at")?.unwrap_or(d.selection.active);
     let color = super::format::color_param(p.get("color")).unwrap_or(Color::Theme(1, 0));
     let shape = Shape {
+        math: Default::default(),
         id,
         kind: ShapeKind::Icon,
         anchor: Anchor { cell: at, dx: 4.0, dy: 4.0, width: size, height: size, mode: anchor_mode_param(p).unwrap_or_default() },