Add a default-on, size-aware foreground PUT admission policy so large or unknown-size PutObject requests are backpressured before body ingest and erasure/RPC fan-out. Preserve the explicit strict gate semantics, including limit=0 as an opt-out, and keep small PUTs on the legacy fast path. Closes rustfs/backlog#2038 Co-authored-by: heihutu <[email protected]>