STRIDE threat model reference

[truthixify]

Extra labels: writing, security
Tier: M (2-4 days) | Type: writing

Context. Paired with contracts issue #14. Once the threat model is drafted in the contracts repo, mirror it in docs as a public reference.

Scope.

• New reference/threat-model.mdx.
• Per-contract STRIDE mapping.
• Residual risks called out separately.
• Cross-link with the audits page.

Acceptance.

• Matches contracts repo THREAT_MODEL.md
• Nav entry
• Reviewed by contracts maintainer

Files. reference/threat-model.mdx (new).