Fix cloning of open-ended default ranges

[itsfuad]

Summary

• preserve nil optional range bounds during default-expression cloning
• retain fresh clone IDs and semantic provenance
• explicitly classify byte and char ownership leaves

Validation

• focused AST, typechecker, ownership, HIR, and LLVM tests
• full Go tests and vet
• bundled compiler check/run for runtime_default_range_clone

Tracking

Progresses #29.

Stacked on the end-to-end remediation PR.