Validates and starts an existing NotStarted sourcing sequence draft owned by the acting user. Drafts created with x-on-behalf-of must be started with the same acting user.

Validation errors and required actions return a structured Blocked startResult and leave the sequence NotStarted and editable. Warnings and informational entries do not block starting. A successful start returns a Started startResult and the sequence's updated status.

Returns sequence_not_found for missing, inaccessible, non-sourcing, or already-started sequences.

Requires the sourcingWrite permission.