docs(changelog): fold #2757 (#2449)

- Folded into the lazy-import entry (#2757): pcapkit.protocols, its layer
  packages, misc.pcap, misc.checksum and pcapkit import their protocol
  classes on first use, pcapkit.protocols.registry builds __proto__ on first
  read, and dir(pcapkit) lists none of the incidental names 1.5.1 listed
  (os, sys, ROOT, warnings, tbtrim), measured on a v1.5.1 export; the ruling
  into the conventions entry.
- No mention of foundation.registry.protocols.protocol_registry, an import
  alias 1.5.1 neither exported nor documented, nor of an import time.
- CHANGELOG.md regenerated; util/changelog_md.py --check passes.

Author: JarryShaw