Add ARC leak checks

[puffball1567]

Summary

• Set the default Nim memory manager to ARC for the repository.
• Add ARC memory-model coverage and a release leak probe.
• Add nimble leak, backed by Valgrind, to fail on definite or indirect leaks.
• Bump the release version for this memory-safety maintenance release.

Validation

• nimble test
• nimble leak with Valgrind: all heap blocks freed; no definite or indirect leaks reported.