navidrome+go-zero: enable residue_prune, add prune_keep_list facts

Phase 1b of the residue-prune rollout (spec: SWE-Milestone
docs/residue-prune-spec.md). Facts live here per the data/code split;
the mechanism ships with the evaluator. Keep-list: navidrome
core/mock_library_service.go (confirmed scaffolding, ~20 graded P2P),
go-zero core/stores/dbtest/sql.go (watch item, SRS-mandated contract).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NhK2BVApsdR7gAKhE3VrNW

navidrome_navidrome_v0.57.0_v0.58.0/metadata.json

@@ -241,5 +241,9 @@
   ],
   "modifiable_test_patterns": [
     "**/agents_plugin_test.go"
+  ],
+  "residue_prune": true,
+  "prune_keep_list": [
+    "core/mock_library_service.go"
   ]
-}
+}

zeromicro_go-zero_v1.6.0_v1.9.3/metadata.json

@@ -398,5 +398,9 @@
         "M014"
       ]
     }
+  ],
+  "residue_prune": true,
+  "prune_keep_list": [
+    "core/stores/dbtest/sql.go"
   ]
-}
+}