Covers the supervisor: the plan-dispatch-synthesize happy path, routing to named workers, isolation of failing, paused, and unroutable subtasks, plan trimming and budget halting flagged as stopped-early, progress events, option validation, and a throwing observer not aborting the run.

## Before writing SPEC.md files

Read https://raw.githubusercontent.com/brillout/sdd/refs/heads/main/sdd.md
