#675 Progress percentage bug: branch task events inflating completed_steps count

When workflows use parallel operators (fork/join patterns), the progress.percentage can exceed 100% because: 1. total_steps only counts top-level tasks in the workflow definition 2. completed_steps counts ALL step_succeeded events from absurd_event_log, including branch task events Example: matrix_fork_join_stress workflow shows 433% (13/3) because: - 3 top-level tasks - 10 fork branch tasks + 3 top-level tasks = 13 step_succeeded events FIXED by filtering completed_steps to only count events where step_name is IN the top-level task_ids set. File: api/blueprints/v1/workflows.py:1477-1481