fix(content): correct verified technical facts across cgroups, namespaces, etcd and k8s sections

- 12.3: daemon.json has no top-level cgroup-driver key; use exec-opts
  native.cgroupdriver (values cgroupfs/systemd) per official dockerd reference
- 12.2: USER namespace is not enabled by default; requires userns-remap
  (aligns with 18.1)
- 14.1: cgroup v1 deprecation and kubelet default-fail happen in v1.35, not
  v1.36 (per kubernetes.io cgroups doc)
- 15_etcd: maintenance window updated after etcd v3.7.0 release (2026-07-08)
  to 3.6/3.7
- 3.9.6: fix dangling sentence order in registry migration note
- tools: add 'from __future__ import annotations' so PEP 604 annotations run
  on Python 3.9 (fixes npm test)

tools/prepare_pdf_sources.py

@@ -8,6 +8,8 @@ and HTML image paths in a temporary copy so PDF builds can embed local assets
 without changing source Markdown used by GitHub or HTML readers.
 """
 
+from __future__ import annotations
+
 import argparse
 import base64
 import mimetypes