Fix bug where immutable entities bypass memoization index during batch processing

[incrypto32]

Immutable entities were bypassing our memoization indexing system. Here's what was happening:

1. Immutable entities were added directly to the batch rows via append_row()
2. This skipped the memoization index that push_row() normally maintains
3. When the system later tried to look up these entities, it couldn't find them despite being present in memory
4. This caused store.get() to return None while the entity existed in cache, triggering assertion panics

Solution

Change append_row() to use self.push_row(row) instead of self.rows.push(row) for immutable entity inserts. This ensures they're properly indexed while preserving the intended performance benefits.

// Before: self.rows.push(row);
// After:  self.push_row(row);