Revise threat model per PMC review (juanpablo-santos) + fix GFM tables

Folds in the PMC's §14 answers as maintainer-confirmed (XML-RPC master vs
master-2.x split, self-registration default+approval, roles, plugin model,
super-linear render + plugin recursion as in-model bugs, Tika-when-enabled);
converts the leading-|| tables to single-pipe Markdown so they render on
GitHub (Copilot review).

Generated-by: Claude Code

Author: potiuk