fix(engine): resolve parallel execution races and improve max steps semantics

- Fix data race in runParallelBranches where first error could be masked
- Improve MaxSteps to allow exactly N steps
- Consolidate Resume validation logic and fix silent overwrite in Graph AddNode/AddEdge
- Add sync.RWMutex to Engine for thread-safe configuration
- Fix P2 issues: Add immediate check for router output, add mutex to hooks example, add context timeout to translation example, and update docs for Checkpointer.Delete

scratch/testmod/go.mod

@@ -0,0 +1,5 @@
+module testmod
+
+go 1.26.2
+
+require github.com/unclesam-ly/GopherGraph v1.1.1 // indirect