AddEntryStreamingAsync now writes to a temp file in the same vault directory,
renames it into place (POSIX rename(2) — atomic on Linux), and updates the
index only after the rename succeeds. A client disconnect or I/O fault during
the write leaves the original backing file intact and the index unchanged; the
temp file is cleaned up best-effort on failure. Fixes the data-corruption
regression on the replace path where a cancelled write could truncate the live
backing file after the index update and FileMode.Create already ran.
Also filters OperationCanceledException from error-level logging in
RegisterResourceStreamingAsync — a normal client disconnect is not an error.
Two tests added to AudioStoreStreamingTests covering cancel and fault on the
replace path.
daniel-c-harvey
beec36a382