--output-dir-flat with --rm: colliding output names overwrite each other and both sources are deleted (exit 0) #4854

Describe the bug
When two inputs map to the same name in the --output-dir-flat folder, FIO_checkFilenameCollisions()
(programs/fileio.c) only prints a warning, and only at display level 2 or above. zstd then compresses both; the
second output overwrites the first (with -f), --rm removes both sources, and the exit code is 0. The first input
is lost.

On Windows it is worse, because the check itself misses collisions:

• it compares names case-sensitively, while the file system is not (d1\a.txt and d2\A.txt produce the same
  output file and no warning at all);
• it splits paths only on PATH_SEP (\ on Windows), while the output name builder also accepts /, so paths
  written with / are compared whole and never collide.

To Reproduce (Windows 11; the first case also applies on any platform)

> mkdir d1 d2 out
> echo first file, d1> d1\x.txt
> echo second file, d2> d2\x.txt
> zstd -f --rm --output-dir-flat out d1\x.txt d2\x.txt
WARNING: Two files have same filename: x.txt
  2 files compressed : ...
> echo %errorlevel%
0
> dir /s /b d1 d2 out
...\out\x.txt.zst          (contains only "second file, d2"; d1\x.txt and d2\x.txt are gone)

Case-only difference, no warning:

> echo first file, d1> d1\a.txt
> echo second file, d2> d2\A.txt
> zstd -f --rm --output-dir-flat out d1\a.txt d2\A.txt
  2 files compressed : ...
> dir /s /b d1 d2 out
...\out\A.txt.zst          (contains "second file, d2"; both sources deleted)

Expected behavior
A collision in the flat output folder is an error before anything is written or removed (or at least, --rm is
refused for a run that has collisions), and the exit code is non-zero. On Windows the comparison ignores case and
treats / and \ alike.

Desktop

• OS: Windows 11 Pro (26200), x64
• Version: v1.5.7; FIO_checkFilenameCollisions() unchanged on dev (2026-10-05)
• Compiler: MSVC 14.51 (Visual Studio 2026 Build Tools), Release
• Build system: build/cmake, Visual Studio generator

Environment caveats

• Found by code review, then reproduced as shown on Windows 11 only (x64), with zstd.exe built from the v1.5.7 source by MSVC 14.51 (Visual Studio 2026 Build Tools) through build/cmake, Win64 Release only. Not tested with MinGW, clang-cl or the official release binaries, nor on Win32.
• tests/playTests.sh was not run (no POSIX shell environment here).
• The functions cited were compared with dev (2026-10-05) by source and are unchanged, but dev itself was not built.
• The analysis was AI-assisted (Claude); every behaviour reported above was reproduced as shown.