Add immutable placement-target selection to Bucket

Buckets could not choose which RGW placement target (and thus durability
profile) backs them, so all data landed on the cluster default. The estate's
radosgw exposes two targets - default-placement (3x replicated) and ec (4+1
erasure-coded) - and archival workloads want ec.

- Validate spec.placementTarget: a DNS-ish pattern, 63-char cap, and a CEL
  self==oldSelf immutability rule (RGW fixes placement at bucket creation and
  cannot move a bucket between targets); make spec.zonegroup immutable too.
- Thread the target into the S3 CreateBucket LocationConstraint via the existing
  helper; an empty zonegroup yields ":<target>", selecting the local zonegroup
  so callers need not name the zonegroup api-name.
- Read the live placement_rule and zonegroup back from the Admin Ops bucket
  stats and surface them: status.placementTarget plus a Placement print column.
- Guard the controller: if a live bucket's placement differs from spec, set an
  Error phase with a PlacementImmutable reason instead of deleting/recreating.
- Cover locationConstraint construction, placement readback (httptest), and the
  placementConflict guard with tests; document targets and immutability in the
  README and add config/samples/06-bucket-ec.yaml.

Claude-Session: https://claude.ai/code/session_015ur3i7D2azsMAWTSVABApv

api/v1alpha1/bucket_types.go

@@ -40,11 +40,29 @@ type BucketSpec struct {
 	// with BucketAccess objects.
 	OwnerRef string `json:"ownerRef"`
 
-	// Zonegroup optionally pins the bucket to a specific RGW zonegroup.
+	// Zonegroup optionally pins the bucket to a specific RGW zonegroup by its
+	// api-name. Empty (the default) uses the cluster's local/master zonegroup, so
+	// PlacementTarget selection works without naming the zonegroup. Immutable:
+	// RGW resolves the zonegroup at bucket creation and cannot move it afterwards.
+	// +kubebuilder:validation:XValidation:rule="self == oldSelf",message="zonegroup is immutable; RGW fixes it at bucket creation"
 	// +optional
 	Zonegroup string `json:"zonegroup,omitempty"`
 
-	// PlacementTarget optionally selects a non-default placement target/pool.
+	// PlacementTarget optionally selects the RGW placement target that backs the
+	// bucket, choosing which pools (and thus replication/erasure profile) store
+	// its data. Empty (the default) uses the owning user's default_placement, or
+	// the zonegroup default. The valid values are cluster configuration, not a
+	// fixed set; on this estate the two configured targets are
+	// "default-placement" (3x replicated) and "ec" (4+1 erasure-coded).
+	//
+	// Immutable: RGW chooses the placement at bucket creation (from the S3
+	// LocationConstraint) and cannot move an existing bucket between placement
+	// targets. Set it on a fresh Bucket; changing it later is rejected, and if a
+	// pre-existing bucket is on a different placement the operator reports an
+	// error instead of recreating it.
+	// +kubebuilder:validation:MaxLength=63
+	// +kubebuilder:validation:Pattern=`^[a-zA-Z0-9]([a-zA-Z0-9._-]*[a-zA-Z0-9])?$`
+	// +kubebuilder:validation:XValidation:rule="self == oldSelf",message="placementTarget is immutable; RGW cannot move a bucket between placement targets"
 	// +optional
 	PlacementTarget string `json:"placementTarget,omitempty"`
 
@@ -100,6 +118,11 @@ type BucketStatus struct {
 	// Owner is the RGW uid that owns the bucket.
 	// +optional
 	Owner string `json:"owner,omitempty"`
+	// PlacementTarget is the placement target RGW actually stores the bucket on,
+	// read back from the live bucket. It makes placement drift (a bucket landing
+	// on a different target than spec requested) visible.
+	// +optional
+	PlacementTarget string `json:"placementTarget,omitempty"`
 	// PolicyPrincipals is the number of extra principals granted via
 	// BucketAccess and reflected in the bucket policy.
 	// +optional
@@ -121,6 +144,7 @@ type BucketStatus struct {
 // +kubebuilder:resource:shortName=bkt
 // +kubebuilder:printcolumn:name="Bucket",type=string,JSONPath=`.status.bucketName`
 // +kubebuilder:printcolumn:name="Owner",type=string,JSONPath=`.status.owner`
+// +kubebuilder:printcolumn:name="Placement",type=string,JSONPath=`.status.placementTarget`
 // +kubebuilder:printcolumn:name="Grants",type=integer,JSONPath=`.status.policyPrincipals`
 // +kubebuilder:printcolumn:name="Adopted",type=boolean,JSONPath=`.status.adopted`
 // +kubebuilder:printcolumn:name="Phase",type=string,JSONPath=`.status.phase`