Verified non-Range server fallback implementation:
- download_to_temp_and_mmap function (http_range.rs)
- TempMmapSource wrapper (source/mod.rs)
- Fallback integration in open_source and open_remote
- Diagnostic emission for REMOTE_NO_RANGE_SUPPORT and REMOTE_INSUFFICIENT_DISK
- Disk space checking with 10% buffer
- RAII cleanup via NamedTempFile
All acceptance criteria verified PASS.