* fix(s3): keep server-side copy data in the bucket collection
UploadPartCopy and SSE-C CopyObject assigned destination volumes against
r.URL.Path, the S3 request URI. The filer derives a bucket's collection
only when the assign path sits under its buckets folder, so an S3 URI
routed copied bytes to the default collection instead of the destination
bucket's. Assign against the destination's real filer path.
* refactor(s3): centralize copy-part path and thread dstPath into SSE-C copy
Extract copyPartLocation so the fast path and writeEmptyCopyPart share one
definition of the .uploads/<id>/<n>_copy.part location. Pass the destination
filer path into copyChunksWithSSEC instead of re-deriving it from the request,
and thread it through key rotation so re-encrypt copies also assign in the
destination bucket's collection.