Add draft project security threat-model document

Adds a draft project-level security threat-model document
(draft-THREAT-MODEL.md) at repo root, improving discoverability
for automated security scanners running against this repository.
The file follows the rubric format used by several other ASF
projects piloting security-model discoverability.

The "draft-" prefix signals this is a proposal for the PMC to
review, correct, or reject — not a finalised maintainer-blessed
model. Every claim carries a provenance tag (documented /
inferred / maintainer) so reviewers can see where each claim
originates; §14 collects open questions for the maintainers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: potiuk