[xiaobai0529]

Summary

When multiple inputs map to the same basename with --output-dir-flat, zstd currently warns only after processing. With --rm, the second output can overwrite the first and both source files are then deleted, while the command exits successfully.

This change detects collisions before processing. If --rm is enabled, the command fails before writing or removing anything. Without --rm, the existing warning and last-output-wins behavior remains.

On Windows, basename comparison now treats / and \ as separators and compares names case-insensitively.

Fixes #4854

Verification

• Default make -j4 zstd build passed.
• The new CLI test covers compression and decompression collisions with --rm, the non-destructive control without --rm, and Windows-only case collisions.
• python3 run.py basic/output_dir_collision.sh basic/output_dir.sh passed both tests.
• Replacing only programs/fileio.c and programs/fileio.h with the pre-fix versions makes the new test fail, proving the test exercises the fix.
• git diff --check passed.

Limitations

• The Windows-only separator and case-folding branches were reviewed and covered by a conditionally executed test, but were not run on a Windows host in this environment.
• The regression test exercises compression and decompression; other modes share the same collision helper but were not separately tested.

AI assistance

AI tools were used to investigate and help write this change. I reviewed the implementation and ran the local verification above.

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