Document adapter versus core runtime boundaries

[enricopiovesan]

Summary

• document the boundary between Traverse core runtime responsibilities and optional adapter responsibilities
• explain why sidecar-like helpers are allowed but not mandatory in Traverse
• add deterministic smoke validation for the adapter-boundary note

Governing Spec

• 001-foundation-v0-1
• 004-spec-alignment-gate

Project Item

• Document adapter responsibilities versus core runtime responsibilities #112

Validation

• bash scripts/ci/adapter_boundaries_smoke.sh
• bash scripts/ci/repository_checks.sh

Closes #112