feat(life-arc): add field requirements for life arcs

Added optional requires clause to life_arc declarations:
  life_arc Baker requires { skill_level: Number } { ... }
Includes new FieldRequirement AST type, requires keyword token,
and parser support for the requires clause.

src/syntax/parser.lalrpop

@@ -338,13 +338,26 @@ OverrideOp: OverrideOp = {
 // ===== Life Arc =====
 
 LifeArc: LifeArc = {
-    "life_arc" <name:Ident> "{" <fields:Field*> <states:ArcState*> "}" => LifeArc {
+    "life_arc" <name:Ident> <reqs:RequiresClause?> "{" <fields:Field*> <states:ArcState*> "}" => LifeArc {
         name,
+        required_fields: reqs.unwrap_or_default(),
         states,
         span: Span::new(0, 0),
     }
 };
 
+RequiresClause: Vec<FieldRequirement> = {
+    "requires" "{" <reqs:Comma<FieldReq>> "}" => reqs,
+};
+
+FieldReq: FieldRequirement = {
+    <name:Ident> ":" <type_name:Ident> => FieldRequirement {
+        name,
+        type_name,
+        span: Span::new(0, 0),
+    }
+};
+
 ArcState: ArcState = {
     "state" <name:Ident> "{" <on_enter:OnEnter?> <fields:Field*> <transitions:Transition*> "}" => ArcState {
         name,
@@ -963,6 +976,7 @@ extern {
         "sub_concept" => Token::SubConcept,
         "concept_comparison" => Token::ConceptComparison,
         "any" => Token::Any,
+        "requires" => Token::Requires,
         "state" => Token::State,
         "on" => Token::On,
         "enter" => Token::Enter,