engine: a typed parameter schema per command (CommandSpec::form)

Field, Kind, Dim and Collection describe a command's JSON parameters: numbers with
unit dimensions, choices, vectors, references to model items, groups, one-of
variants and rows. prefill, switch_variant and problems serve forms; json_schema
gives the same as JSON Schema. No command is described yet (form: None).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/edit.rs

@@ -36,6 +36,7 @@ pub const UNDO: CommandSpec = CommandSpec {
     id: "edit.undo",
     label: "Undo",
     params: r#"{"count"?: 1} — takes back the last command that changed the model (it leaves the journal and can be redone); `count` takes back that many, as far as the history reaches"#,
+    form: None,
     kind: CommandKind::Session,
     run: |s, p| repeat(s, p, Session::undo),
 };
@@ -44,6 +45,7 @@ pub const REDO: CommandSpec = CommandSpec {
     id: "edit.redo",
     label: "Redo",
     params: r#"{"count"?: 1} — runs the last undone command's change again; `count` redoes that many, as far as there are undone commands"#,
+    form: None,
     kind: CommandKind::Session,
     run: |s, p| repeat(s, p, Session::redo),
 };