Function perform_migration_impl 

pub fn perform_migration_impl(
    source: &Database,
    dest: &Database,
    project_hint: Option<&str>,
    all: bool,
    dry_run: bool,
) -> Result<MigrationStats, CoreError>

Copies memories from source into dest.

Transaction semantics: dest.bulk_insert_memories wraps the inserts in a single SQLite transaction — either all queued rows commit or none do (all-or-nothing). Duplicates MUST therefore be filtered out before the bulk call; otherwise a single duplicate ID rolls back the entire batch.