fix: derive contingent change signal from the sync diff

src/Model/ContingentSyncResult.php

@@ -13,19 +13,24 @@ readonly class ContingentSyncResult
         public bool $successful,
         public bool $changed,
         public int $added,
+        public int $extended,
         public int $updated,
         public int $removed,
         public ?string $error = null,
     ) {
     }
 
-    public static function synced(bool $changed, int $added, int $updated, int $removed): self
+    /**
+     * $added counts days newly stored within the horizon the previous run already reached;
+     * $extended counts days beyond it, which is the rolling window growing rather than a change.
+     */
+    public static function synced(bool $changed, int $added, int $extended, int $updated, int $removed): self
     {
-        return new self(true, $changed, $added, $updated, $removed);
+        return new self(true, $changed, $added, $extended, $updated, $removed);
     }
 
     public static function failed(string $error): self
     {
-        return new self(false, false, 0, 0, 0, $error);
+        return new self(false, false, 0, 0, 0, 0, $error);
     }
 }