Fail loudly on sibling filename collisions

[argonui]

Summary

getAGoodFileName() returns <sanitized nickname>.<guid>, used both as the on-disk filename and as the object's identity in the *_order arrays, with no uniqueness check. Two siblings that collapse to the same name — a shared GUID inside a container, or nicknames differing only in stripped punctuation — caused the second WriteObj to silently overwrite the first.

The maintainer chose to fail loudly on collision rather than disambiguate.

Changes

• Add checkFilenameCollisions([]*objConfig) which returns an error naming the colliding filename and the two GUIDs involved.
• Invoke it where siblings sharing a directory are enumerated:
  • Printer.PrintObjectStates — root objects (top-level objects/ directory).
  • parseFromJSON — contained objects and states together, since both are written into the same subdirectory.
• The check is scoped strictly to siblings sharing a directory, where the overwrite actually occurs.
• Add unit tests: TestPrintObjectStatesCollision (shared GUID and stripped-punctuation cases) and TestContainedObjectCollision.

go build ./..., go vet ./..., and go test ./... all pass; no existing fixtures collide.

Fixes #107