fix: enforce retention authorization and immutable version writes

Close SN-2026-015 by requiring PutObjectRetention with request-derived
condition keys before reading bucket or object state. Weakening unexpired
GOVERNANCE retention also requires the bypass header and permission. Treat
retention removal as zero remaining days and avoid duration saturation for
distant dates.

Close SN-2026-016 by requiring replication trust for caller-selected target
version IDs and checking persisted multipart targets and locked null
versions under the destination write lock, including across storage pools.
Preserve versioning transitions, trusted replication and batch Snowball
version IDs, and fail closed when lock state cannot be read.

Add authorization, concurrent lock-update, multipart transition, cross-pool,
configuration-failure and retention-boundary regression coverage. Validate
with make test, focused race tests and the original independent probes.

Developed with Codex (Astra).

Co-authored-by: Codex <codex@openai.com>
Signed-off-by: Feng Ruohang <rh@vonng.com>
(cherry picked from commit fe3935476b929e03193d7496a7448d760e635096)

cmd/erasure-server-pool-multipart-http_test.go

@@ -265,6 +265,7 @@ func TestPoolsMultipartConditionalLatestVersionAndCallbackOnce(t *testing.T) {
 			if explicit {
 				current = putConsistencyObject(t, z, bucket, object, 1, "latest-other-version", ObjectOptions{Versioned: true, MTime: UTCNow().Add(-time.Minute)})
 			}
+			opts.VersionID = ""
 			id, parts := multipartConditionUpload(t, z, bucket, object, 1, opts)
 			called := 0
 			opts.CheckPrecondFn = func(oi ObjectInfo) bool { called++; return oi.ETag != current.ETag }