feat(replication): SSE-C ciphertext passthrough replication (#5898)

Complete the encrypted-object replication series (backlog#1783, PR-C of
3, after #5872 and #5885): SSE-C objects replicate as ciphertext
passthrough — the source holds no customer key, so the stored bytes and
their encryption metadata travel verbatim and the replica decrypts only
with the original customer key, single-part and multipart.

- Sender: SSE-C objects read raw (raw_data_movement_read), transfer at
  ciphertext size, and range multipart parts over stored part sizes.
- Receiver: authorized replication PUTs restore the stored SSE-C keys
  from the transport headers (exact lowercase forms - the read-path
  check is case-sensitive), set ObjectOptions.preserve_ciphertext, and
  skip compression, bucket-default SSE, and sse_encryption behind one
  restore-derived gate. Multipart uses an internal session marker to
  store parts verbatim and strips it on complete.
- Convergence: the replication HEAD sends
  x-rustfs-source-replication-check; the target authorizes it as
  ReplicateObjectAction and skips SSE-C read validation for that
  request only, so keyless convergence HEADs see etag/size/mtime
  instead of 400 and SSE-C replicas stop re-driving forever.
- e2e: SSE-C contract flips to a key-gated readable replica (no-key and
  wrong-key GETs fail - the direct silent-plaintext detector); new
  multipart passthrough contract with ETag/marker/stability assertions.

crates/ecstore/src/set_disk/ops/multipart.rs

@@ -1897,6 +1897,13 @@ impl crate::storage_api_contracts::multipart::MultipartOperations for SetDisks {
             }
         }
 
+        // The SSE-C passthrough session marker is upload-scoped; drop it from
+        // the completed object's metadata.
+        rustfs_utils::http::metadata_compat::remove_str(
+            &mut fi.metadata,
+            rustfs_utils::http::SUFFIX_REPLICATION_PRESERVE_CIPHERTEXT,
+        );
+
         if checksum_type.is_set() {
             checksum_type
                 .merge(rustfs_rio::ChecksumType::MULTIPART)

crates/ecstore/src/set_disk/ops/object.rs

@@ -1233,6 +1233,16 @@ impl SetDisks {
                 }
             }
 
+            // SSE-C replication carries the source object's sealed checksum
+            // out of band; store it verbatim like the multipart path does.
+            if let Some(cssum) =
+                rustfs_utils::http::get_header_map(&user_defined, rustfs_utils::http::SUFFIX_REPLICATION_SSEC_CRC)
+                && !cssum.is_empty()
+            {
+                fi.checksum = base64_simd::STANDARD.decode_to_vec(&cssum).ok().map(bytes::Bytes::from);
+                rustfs_utils::http::remove_header_map(&mut user_defined, rustfs_utils::http::SUFFIX_REPLICATION_SSEC_CRC);
+            }
+
             if fi.checksum.is_none()
                 && let Some(content_hash) = data.as_hash_reader().content_hash()
             {