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>

spec/belongs_to_spec.rb

@@ -3,7 +3,7 @@ require 'spec_helper'
 describe "BelongsTo" do
   context "with associations" do
     context "with belongs_to" do
-      it "generate a view for the association" do
+      it "generate a view for the association", :couchdb_only do
         #Post.respond_to?(:association_post_belongs_to_user).should be_present
         Post.should respond_to :association_post_belongs_to_user
       end