fix(cli): --output-dir-flat collisions with --rm delete a source whose output was overwritten #623

Problem

With --output-dir-flat, two inputs whose outputs land on the same name in the flat directory overwrite each other. With -f --rm both sources are then removed and the run exits 0, so the first input is lost. The collision is reported only after every input has been processed, as a warning at display level 2.

On macOS (and Windows) the file system ignores case, while the collision check compares names byte for byte: d1/a.txt and d2/A.txt write one out/a.txt.zst with no warning at all.

Reproduced on main on throwaway copies:

mkdir d1 d2 out
printf 'first\n' > d1/x.txt; printf 'second\n' > d2/x.txt
structured-zstd -f --rm --output-dir-flat out d1/x.txt d2/x.txt   # exit 0
# d1, d2 empty; out/x.txt.zst decodes to "second"

Upstream reports the same for the reference command: meta/zstd#4854 (fix proposed in meta/zstd#4855).

Fix

• Before anything is written or removed, derive every input's output path and compare the names in the flat directory. With --rm, a collision is an error and the run stops with both sources in place. Without --rm, the warning and last-output-wins behaviour stay.
• While the run writes, remember the identity (device and inode, or the Windows file ID) of every output it creates. With --rm, an input whose output is a file this run already wrote is refused and kept. This covers the names a byte comparison cannot judge (case or Unicode normalisation ignored by the file system) without guessing per platform, so a case-sensitive volume is never refused for names that are really distinct.

Acceptance

• -f --rm --output-dir-flat out d1/x.txt d2/x.txt exits non-zero, writes nothing and keeps both sources.
• A case-only difference on a case-insensitive file system exits non-zero, keeps the first archive and the second source.
• Without --rm the run still writes the last output and warns, as the reference command does.
• Regression tests on temporary copies cover compression and decompression.