Avoid data loss in dedup_public.py when a link cannot be created

[Valyrian-Code]

Problem

scripts/dedup_public.py replaces a file that is byte-identical to a base-language file with a link to that canonical copy. It did so by unlinking the original before creating the replacement:

p.unlink()
if args.mode == 'symlink':
    os.symlink(rel_target, p)
else:
    os.link(canonical, p)

If os.symlink/os.link then fails (disk, permissions, cross-device for hardlinks, etc.), the file is left deleted with no replacement and is lost permanently. The except only prints; it cannot restore the file. This script runs during the production deploy (build_prod_incremental.sh / build_prod_per_lang.sh), so a single failure mid-run drops a published asset.

Fix

Create the link at a temporary path first, then atomically os.replace it over the original. A failed link now leaves the original file untouched.

Test

Adds test/test_dedup_public.py (builds on the pytest harness from #1015):

• injects an os.symlink failure and asserts the original file survives intact;
• a success case asserting the duplicate becomes a symlink resolving to the canonical bytes, with no temp file left behind.

Reverting the fix makes the first test fail (the file is lost), confirming it has teeth.

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[Xpirix]

Great fix @Valyrian-Code , thanks! LGTM