Function compute_migration 

pub fn compute_migration(
    old_schema: &SymbolTable,
    new_schema: &SymbolTable,
    config: &MigrationConfig,
) -> Result<SchemaMigrationPlan, MigrationError>

Compute the full migration plan needed to go from old_schema to new_schema.

Steps:

1. Build snapshots of both schemas.
2. Detect predicate additions, removals, and arity changes.
3. Detect domain additions and removals.
4. Detect variable/rule additions and removals.
5. Optionally resolve renames by similarity.
6. Synthesise SchemaMigrationSteps.
7. Enforce allow_breaking_changes policy.