Unsupported remote blob materialization errors now point to row-ID
selection, fetch_blobs and fetch_blob_files. Document descriptor
structs, fetched bytes and Function BlobReader inputs, and show a
bounded subset-copy recipe that keeps the source unchanged between
selection and fetching.
Adds guidance assertions and copy coverage preserving payloads, empty
blobs and nulls.
No new blob materialization API is introduced.
Formatting applied; no local builds or tests were run. CI will execute
the regression coverage.
Jack Ye
481e82293e