fix(dag): cancel orphan child after start failure

[LeXwDeX]

Summary

• cancel a child Session when NodeStarted persistence fails after Session creation
• retain transition-rejection behavior and avoid duplicate terminal events
• add a deterministic regression test at the public spawn seam

Verification

• bun run test:dag-core: Core 90, OpenCode DAG 402, Schema 3, TUI 50
• generated SDK unchanged
• packages/opencode bun typecheck
• repository pre-push typecheck: 29/29

Closes #212