fix: guarantee parser advance to avoid infinite recursion

compiler/src/parser_base.ts

@@ -38,6 +38,11 @@ export class TokenStream {
     this.loopGuard = 0;
   }
 
+  /** Current position in the token stream (used to detect lack of progress during error recovery) */
+  position(): number {
+    return this.pos;
+  }
+
   peek(offset: number = 0): Token {
     return this.tokens[this.pos + offset] ?? this.tokens[this.tokens.length - 1];
   }

compiler/src/parser_recovery.ts

@@ -57,6 +57,7 @@ export class RecoveringParser {
     const systems: AST.SystemDeclNode[] = [];
 
     while (!this.s.check(TokenKind.EOF)) {
+      const before = this.s.position();
       try {
         systems.push(this.parseSystemDecl());
       } catch (e) {
@@ -67,6 +68,11 @@ export class RecoveringParser {
           throw e;
         }
       }
+      // Guarantee forward progress: if neither parsing nor synchronization
+      // consumed a token, skip one to avoid an infinite recovery loop.
+      if (this.s.position() === before && !this.s.check(TokenKind.EOF)) {
+        this.s.advance();
+      }
     }
 
     if (systems.length === 0 && this.errors.length === 0) {
@@ -107,6 +113,7 @@ export class RecoveringParser {
 
     const declarations: AST.DeclarationNode[] = [];
     while (!this.s.check(TokenKind.RBrace) && !this.s.check(TokenKind.EOF)) {
+      const before = this.s.position();
       try {
         declarations.push(this.parseDeclaration());
       } catch (e) {
@@ -117,6 +124,15 @@ export class RecoveringParser {
           throw e;
         }
       }
+      // Guarantee forward progress to avoid an infinite recovery loop when
+      // synchronization lands on a token the body loop cannot consume.
+      if (
+        this.s.position() === before &&
+        !this.s.check(TokenKind.RBrace) &&
+        !this.s.check(TokenKind.EOF)
+      ) {
+        this.s.advance();
+      }
     }
 
     this.s.expect(TokenKind.RBrace, "system body close");