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

scratch/testmod/go.sum

@@ -0,0 +1,2 @@
+github.com/unclesam-ly/GopherGraph v1.1.1 h1:1jou9QFtsom/eK69Evx+J3HLkZTjKjnuEuRvkir8qi4=
+github.com/unclesam-ly/GopherGraph v1.1.1/go.mod h1:+99WeqLyTIYfOs2OSob/SQ3Pnw+R7pxW8+o6D7S4m6g=