feat: sparse writes with range_upload, zero-download write path #974

Job	Run time
Lint & Unit Tests	1m 26s
POSIX Compliance (pjdfstest)	1m 35s
Smoke Tests (FUSE + NFS)	4m 17s
Benchmarks	2m 19s
fsx (data integrity)	4m 0s
xfstests (filesystem exerciser)	14m 1s
	27m 38s

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