v0.30.0 — Plan lifecycle validation & render-stack removal

Two threads of plan-tooling hardening that finish work the previous milestones deliberately left open. First, teach plan validate to catch lifecycle drift — statuses that contradict the terminal evidence around them (SPEC-119), the symmetric counterpart to the one-directional lifecycle checks already in place. Second, remove the bespoke plan build / plan serve render stack (SPEC-120) that v0.16.0 deprecated but kept alive — with the SPEC-071 plan-site transition shipped, a plan site is now an ordinary refrakt site, so the parallel SSG can go.