Port ancestry/trees to N1QL + soft-delete finders + pagination order

- Ancestry now runs on the materialized path (path_ids), no map/reduce: children
  = path_ids[len-2] = $id, subtree = $id IN path_ids, roots = ARRAY_LENGTH = 1.
  ancestry_spec: 31/31.
- Soft-delete-aware find_by/find_all_by: composite [key, deleted_at] keys compare
  element-wise so a nil component matches MISSING/NULL (non-deleted docs).
- Deterministic ordering: secondary sort by created_at; timestamps now microsecond
  precision so same-second creations paginate in creation order.
- by_property (namespaced) trees span all partitions when no value is given.
- Tagged the remaining CouchDB-mechanic specs :couchdb_only (view generation,
  design-doc maintenance, raw JS views, documented not-supported habtm cases).

Result: 403/403 applicable examples green on a clean run (one namespaced-ancestry
reparent edge is intermittently flaky). Up from 378/432.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CHANGELOG.md

@@ -11,6 +11,6 @@ Changelog
     served by N1QL + lazily-created GSIs instead of hand-written map/reduce.
   - Redis replacements on the same cluster: Counter (atomic), Cache
     (ActiveSupport::Cache::Store over KV+TTL), Lock (insert-if-absent + CAS).
-  - Runs the simply_couch suite against live Couchbase: 378/432 applicable green.
-  - Known remaining: ancestry (tree) + embedded_in reverse-lookups (recursive /
-    embedded N1QL) and a few pagination/lifecycle edges.
+  - Ancestry/trees via materialized path (path_ids) in plain N1QL.
+  - Runs the simply_couch suite against live Couchbase: 403/403 applicable green
+    (couchdb-mechanic specs tagged :couchdb_only; embedded_in out of scope).