docs: establish OPSEC threat model and gap register

[mark-e-deyoung]

Summary

• document assets, adversaries, trust zones, critical seams, and evidence requirements
• identify confirmed code conflicts in channel crypto, transport policy, trusted updates, credentials, logs, discovery, deployment, and supply chain
• define source/binary/installer/runtime automation, a NIST covering-array matrix, and formal invariants
• mark the older security audit as historical rather than a current attestation

Tracking

• Security: enforce authorization and exposure policy across transports #300 transport authorization/exposure
• Security: replace custom channel crypto and prove auth interoperability #301 cryptographic channel/authentication
• Security: require signed, verified updates before installer execution #302 trusted update execution

Validation

• code findings cross-checked against current implementations
• primary references linked in the document
• git diff --check`n
  Stacked on ci: enforce Windows/Wine compatibility policy #299. Documentation only; no security claim is promoted to verified by this PR.