feat(ecstore): runtime-probed io_uring read backend, gray-off by default (backlog#1104) (#4632)

feat(ecstore): add runtime-probed io_uring read backend, gray-off by default (rustfs/backlog#1104)

Wire the standalone rustfs-uring crate (https://github.com/rustfs/uring) into
LocalIoBackend as an opt-in read backend. When RUSTFS_IO_URING_READ_ENABLE is
set AND the per-disk io_uring probe succeeds, positioned reads go through the
cancel-safe UringDriver; otherwise — default, or on a restricted host, or on
any per-read driver error — reads fall back to StdBackend byte-for-byte, so the
default build is unchanged.

- Cargo.toml: rustfs-uring as a Linux-only git dependency (pinned rev). The
  guard scripts/check_no_tokio_io_uring.sh allows an explicit io-uring
  integration; only the tokio "io-uring" runtime feature is banned.
- UringBackend mirrors StdBackend::pread_bytes's resolution/access/bounds
  preamble and only swaps the raw byte read; the other three trait methods
  delegate to the inner StdBackend.
- Backend selection at LocalDisk construction via build_local_io_backend.
- Differential test uring_backend_reads_match_std: with the flag set, every
  positioned read returns byte-identical data (driver-served when io_uring is
  available, StdBackend fallback otherwise).

Verified: cargo check -p rustfs-ecstore on Linux; the differential test passes
under real io_uring (seccomp=unconfined). The runtime errno degradation latch,
per-disk probe cache, and productionization are tracked in
rustfs/backlog#1101/#1102.

Co-authored-by: heihutu <heihutu@gmail.com>

crates/ecstore/Cargo.toml

@@ -140,6 +140,13 @@ aws-smithy-http-client.workspace = true
 # Observability and Metrics
 metrics = { workspace = true }
 
+# Runtime-probed io_uring read backend (backlog#1104). Linux-only, git
+# dependency until rustfs-uring is production-ready and published. The guard
+# scripts/check_no_tokio_io_uring.sh allows an explicit io-uring integration;
+# only the tokio "io-uring" runtime feature is banned.
+[target.'cfg(target_os = "linux")'.dependencies]
+rustfs-uring = { git = "https://github.com/rustfs/uring", rev = "39018c09cde98b380658f1a1511349a3574daddf" }
+
 [dev-dependencies]
 tokio = { workspace = true, features = ["rt-multi-thread", "macros", "test-util"] }
 criterion = { workspace = true, features = ["html_reports"] }