chore: align module folder names to platform naming convention

[mdproctor]

Summary

Apply the platform module naming convention (PP-20260508-5c0e4b): short folder names, fully-qualified artifactIds.

Folder renames (git mv + root pom module entries)

Current	Should be
casehub-blackboard	blackboard
casehub-engine-common	common
casehub-ledger	ledger
casehub-persistence-memory	persistence-memory
casehub-persistence-hibernate	persistence-hibernate
casehub-resilience	resilience
casehub-testing	testing
casehub-work-adapter	work-adapter
engine	runtime

No artifactId changes — only directory names and root pom <module> entries.

References

Protocol: docs/protocols/maven-submodule-folder-naming.md (PP-20260508-5c0e4b)