v2.3.1

Chores

• helm: Bump appVersion to 2.3.1 (anvil dump + size units)
• docs: Move internal design notes out of git; de-reference them in code
• docs: Move OBOX.md to docs_internal; drop upstream ref in a config comment
• docs: Move TODO.md internal; scrub remaining upstream refs & dead links

Documentation

• Neutralize incidental upstream-product references
• pop3: Fix stale session-lock filename (dovecot- → yarilo-pop3-session.lock)
• helm: Drop upstream config-file name from username_hash comment

Features

• config: Human-readable units for the remaining byte-size fields
• anvil: Yarctl anvil dump — admin state introspection