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

config/samples/06-bucket-ec.yaml

@@ -0,0 +1,18 @@
+# A bucket placed on the erasure-coded (4+1) placement target instead of the
+# default 3x-replicated pool. Good for bulk/archival data where capacity matters
+# more than the extra replica.
+#
+# placementTarget is immutable: RGW chooses the placement at bucket creation and
+# cannot move an existing bucket between targets, so it can only be set on a
+# fresh Bucket. The operator reports the live placement in status.placementTarget.
+apiVersion: ceph.unkin.net/v1alpha1
+kind: Bucket
+metadata:
+  name: raw-archive
+  namespace: default
+spec:
+  bucketName: raw-archive
+  ownerRef: app-owner
+  # Cluster-configured placement target. On this estate: "default-placement"
+  # (3x replicated) or "ec" (4+1 erasure-coded).
+  placementTarget: ec