DIFF TABLE does not support HEAP-organized tables

Description

The DIFF TABLE statement fails when executed on a table created with ORGANIZATION = HEAP.

Steps to Reproduce

1. Create a source table with ORGANIZATION = HEAP:

   create table t_heap_src (
     id int,
     name varchar(50),
     score int
   ) ORGANIZATION = HEAP;
   

2. Attempt to execute a DIFF TABLE statement (the specific fork t... command from the original report is incomplete).
3. The operation fails with an error.

Impact

Users cannot use the DIFF TABLE feature to compare or merge data for tables using the HEAP storage organization.

No source file, test, or entry point is named. Start by reproducing the failure with the provided HEAP table and a complete DIFF TABLE statement, then trace the database implementation and existing DIFF TABLE tests; done means DIFF TABLE works for HEAP-organized tables and the regression is covered.