v2.3.1

v2.3.1 (Phase C closure)

• rtk validate — local quality gate (dev-gate.sh or cargo workspace)
• e2e_smoke.sh in CI Linux
• Dogfood: scripts/setup-dogfood.sh, docs/DOGFOOD.md
• docs/PLAN_CLOSURE.md — operational closure plan
• Homebrew formula URLs + sha256 for v2.3.1