d3546305f6
RFC 4918 §9.7.1 forbids partial updates on PUT; this adds PATCH as the supported mechanism instead, via an X-Update-Range header (bytes=<start>-<end> or append). Reuses the existing CAS/dedup pipeline by splicing the request body between the file's untouched prefix/suffix byte ranges and re-ingesting as one continuous stream, so unedited chunks dedup for free.