DOC: internals: add a threat model definition

DOC: internals: add a threat model definition

Add doc/internals/threat-model.txt describing what does and does not
qualify as a security vulnerability in HAProxy so that reporters and
developers have a common understanding of the threat model, and make it
clear that anything non-critical should be handled in the open and
not hidden behind embargoes.

The document lists assets to protect, what constitutes an attack, what
are the mitigations in place, and the severity ordering of various
risks. This may in the long term also help developers make better
choices of default settings and option names, and may also justify
changing default settings over time when modern operating systems
bring new possibilities.

A section also lists some invariants and defaults in an attempt to
limit the risk of reporting theoretical issues that are technically
impossible to happen in the field.

This is an initial version meant to be refined as cases arise. It
was incrementally designed and cross-checked with the help of three
independent LLMs (Qwen, Gemini and Claude) until each correctly
classified a set of sample reports against it. In the current state
they do not raise any residual ambiguities anymore.