chore: add phase0 architecture guardrails and docs taxonomy

[Lythaeon]

Summary

• Implemented Phase 0 architecture guardrails for the hexagonal + vertical-slice migration.
• Reorganized docs into a typed taxonomy (guides, assets, architecture/{ard,adr,srs,rfc,patterns}) and updated all links.
• Stabilized the flaky gRPC runtime test path before merge.

Changes

• Added architecture guardrail artifacts and taxonomy docs:
  • docs/architecture/adr/ADR-0001-hexagonal-vertical-slices.md
  • docs/architecture/ard/ARCHITECTURE_RISKS_HEXAGONAL_PLAN.md
  • docs/architecture/ard/ARCHITECTURE_BASELINE_METRICS.md
  • docs/architecture/README.md + category readmes/templates
• Added architecture enforcement tooling:
  • scripts/check_architecture.sh
  • cargo make architecture-check in Makefile.toml
  • CI integration in .github/workflows/pr.yml and .github/workflows/release.yml
• Reorganized docs and assets:
  • moved usage/advanced docs to docs/guides/
  • moved screenshots/charts to docs/assets/images and docs/assets/charts
  • updated references in README.md, CHANGELOG.md, AGENTS.md, and docs indexes
• Hardened gRPC runtime/test behavior:
  • added .no_proxy() for gRPC client construction
  • improved grpc/grpcs URL scheme normalization fallback
  • skipped transport preflight for gRPC protocols
  • adjusted gRPC mock/test expectations and increased test timeout budgets

Checks

• cargo make format
• cargo make clippy
• cargo make test
• cargo make architecture-check
• cargo make test-wasm (not applicable)
• cargo make audit (not applicable)
• cargo make deny (not applicable)