v0.6.4

What changed

Docs/metadata hardening round.

• Fix stale test counts in docs (411/424 → 442)
• Update figures/generate_header.py version string
• Complete README architecture tree (3 missing files)
• Add scpn-phase-orchestrator to Related Repositories
• Update SECURITY.md supported versions (0.6.x)
• Add pip ecosystem to dependabot monitoring
• Add v0.6.1–v0.6.4 entries to changelogs
• Version bump 0.6.2 → 0.6.4 across all metadata files