fix: support comma-separated behavioral properties without repeating 'is'

Allows `is pure, commutative` syntax in addition to `is pure is commutative`.
After the initial `is`, comma-separated properties no longer require repeating
the `is` keyword (though it's still accepted for compatibility).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/parser.rs

@@ -879,7 +879,8 @@ impl Parser {
         Ok(effects)
     }
 
-    /// Parse behavioral properties: is pure, is total, is idempotent, etc.
+    /// Parse behavioral properties: is pure, total, idempotent, etc.
+    /// Supports: `is pure`, `is pure is total`, `is pure, total`, `is pure, is total`
     fn parse_behavioral_properties(&mut self) -> Result<Vec<BehavioralProperty>, ParseError> {
         let mut properties = Vec::new();
 
@@ -901,9 +902,15 @@ impl Parser {
             let property = self.parse_single_property()?;
             properties.push(property);
 
-            // Optional comma for multiple properties: is pure, is total
-            if self.check(TokenKind::Comma) {
-                self.advance();
+            // After first property, allow comma-separated list without repeating 'is'
+            while self.check(TokenKind::Comma) {
+                self.advance(); // consume comma
+                // Allow optional 'is' after comma: `is pure, is total` or `is pure, total`
+                if self.check(TokenKind::Is) {
+                    self.advance();
+                }
+                let property = self.parse_single_property()?;
+                properties.push(property);
             }
         }