test(12.1): owner-confirmed expected root manifest

- Create backend/tests/fixtures/cloud/nextcloud_expected_root.json with
  owner_confirmed: true and corrected 10-item name/kind manifest
- Add test_nextcloud_expected_root_manifest: exact set and kind equality
  against fixture (T-12.1-19 — count-only acceptance blocked)
- Update 12.1-VALIDATION.md: manifest status OWNER CONFIRMED, add
  exact-acceptance row to Nyquist matrix

backend/tests/fixtures/cloud/nextcloud_expected_root.json

@@ -0,0 +1,16 @@
+{
+  "owner_confirmed": true,
+  "provenance": "Owner reconciled 2026-06-22 — three fixture names corrected from prior candidate manifest; no provider content exposed",
+  "items": [
+    {"name": "Documents",                      "kind": "folder"},
+    {"name": "docuvault",                      "kind": "folder"},
+    {"name": "Photos",                         "kind": "folder"},
+    {"name": "Templates",                      "kind": "folder"},
+    {"name": "Nextcloud.png",                  "kind": "file"},
+    {"name": "Nextcloud Manual.pdf",           "kind": "file"},
+    {"name": "Nextcloud intro.mp4",            "kind": "file"},
+    {"name": "Readme.md",                      "kind": "file"},
+    {"name": "Reasons to use Nextcloud.pdf",   "kind": "file"},
+    {"name": "Templates credits.md",           "kind": "file"}
+  ]
+}