Engine: each command lists what it needs, and execute checks it first

CommandSpec gains `requires: &[Requirement]` (crates/engine/src/requirement.rs): an open
design, geometry, nodes, a mesh, something to analyse, materials and sections for the
elements, sets, the definition an edit or delete acts on, a step, a result. Session::execute
checks them before running, so a command missing one fails at once with no partial work,
naming each missing piece and the toolbar entry that provides it ("Needs a material —
Properties › Material"). Requirements are real prerequisites (the model's own reference
checks and model.check's rules), never a workflow order. engine.commands / list_commands
list them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/edit.rs

@@ -38,6 +38,7 @@ pub const UNDO: CommandSpec = CommandSpec {
     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: Some(super::forms::COUNT),
     kind: CommandKind::Session,
+    requires: &[],
     run: |s, p| repeat(s, p, Session::undo),
 };
 
@@ -47,6 +48,7 @@ pub const REDO: CommandSpec = CommandSpec {
     params: r#"{"count"?: 1} — runs the last undone command's change again; `count` redoes that many, as far as there are undone commands"#,
     form: Some(super::forms::COUNT),
     kind: CommandKind::Session,
+    requires: &[],
     run: |s, p| repeat(s, p, Session::redo),
 };