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/lexer.rs

@@ -37,6 +37,8 @@ pub enum Token {
     ConceptComparison,
     #[token("any")]
     Any,
+    #[token("requires")]
+    Requires,
     #[token("state")]
     State,
     #[token("on")]