docs(agent): add Gemstack topology migration

[arvarik]

Summary

Add Gemstack topology-aware configuration to the .agent/ documentation suite. All changes are additive only — no existing content modified, deleted, or renumbered.

Key Changes

• ARCHITECTURE.md: Inserted §0 Project Topology declaration ([frontend, backend]) with instructions to read frontend.md and backend.md Gemstack profiles
• TESTING.md: Added 3 topology-specific matrices:
  • Backend Route Coverage Matrix (N/A — eero-stats is a polling daemon, no HTTP routes)
  • Frontend Component State Matrix (pre-populated with all 7 Grafana dashboard sections)
  • ML/AI Evaluation Thresholds (N/A)
• STATUS.md: Appended 2 tracking sections:
  • Stub Audit Tracker (no active frontend stubs; test-scoped mocks documented)
  • Prompt Versioning Changelog (N/A)

Testing

• Docs-only change — no code modified
• Verified all 3 files preserve existing content structure and section numbering
• Tables pre-populated from actual codebase analysis (Grafana dashboard sections, mock scan)