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/model.rs

@@ -2,10 +2,11 @@ use fieldcraft_model::{Severity, UnitSystem};
 use serde_json::{Value, json};
 
 use super::{listed, name};
+use crate::requirement::Requirement;
 use crate::{CommandKind, CommandSpec, EngineError, Result, Session};
 
 pub const INFO: CommandSpec =
-    CommandSpec { id: "model.info", label: "Model Info", params: "{}", form: Some(&[]), kind: CommandKind::Model, run: info };
+    CommandSpec { id: "model.info", label: "Model Info", params: "{}", form: Some(&[]), kind: CommandKind::Model, requires: &[], run: info };
 
 pub const UNITS: CommandSpec = CommandSpec {
     id: "model.units",
@@ -13,11 +14,12 @@ pub const UNITS: CommandSpec = CommandSpec {
     params: r#"{"system": "m-kg-s-N-Pa|mm-t-s-N-MPa|mm-kg-ms-kN-GPa|mm-g-ms-N-MPa|in-lbf-s-psi|unspecified"} — labels the system the model's numbers are in; nothing is converted"#,
     form: Some(super::forms::UNITS),
     kind: CommandKind::Model,
+    requires: &[],
     run: units,
 };
 
 pub const CHECK: CommandSpec =
-    CommandSpec { id: "model.check", label: "Check Model", params: "{}", form: Some(&[]), kind: CommandKind::Model, run: check };
+    CommandSpec { id: "model.check", label: "Check Model", params: "{}", form: Some(&[]), kind: CommandKind::Model, requires: &[], run: check };
 
 pub const SET_RESOLVE: CommandSpec = CommandSpec {
     id: "model.set.resolve",
@@ -25,6 +27,7 @@ pub const SET_RESOLVE: CommandSpec = CommandSpec {
     params: r#"{"name": "root"} — the set's nodes, elements and element faces on the current mesh (the first 100 of each); fails naming every member that is not on the mesh"#,
     form: Some(super::forms::SET_RESOLVE),
     kind: CommandKind::Model,
+    requires: &[Requirement::Set],
     run: set_resolve,
 };