R1-001: Ratify Deterministic Snapshot Kernel ADR and architecture decision log

[1sgtpepper]

Summary

Ratifies the Deterministic Snapshot Kernel ADR, links it from the architecture overview and site navigation, and records current versus planned ownership without claiming unfinished host migrations are complete.

Linked issue

Fixes #133.

PR Checklist

• Linked issue checked.
• One layer or one mechanical change.
• Behavior/schema/golden impact checked.
• Tests/docs updated.
• Local checks run.
• Keep draft until required CI is green.

Release target

R1 foundations

Single layer changed

• docs/governance

Behavior impact

• No user-visible behavior change

Tests and docs

• Docs updated
• Site navigation updated
• Package manifest updated

Dependency checklist

• Checked docs/contributing/pr-dependency-checks.md and updated every dependent artifact.

Local checks

pytest tests/test_pack_manifest.py -q
mkdocs build --strict
git diff --check

All passed.