bf53a8eaa8
The v2.6.0 pi-devbox CLI-parity additions (pandoc + graphviz + tealdeer, ~200 MB on the base layer) consumed nearly all the ~250 MB headroom that v1.16.2 restored. v2.6.0 passed 2750/3450 but with a slim margin. Restore ~250 MB headroom so the guardrail catches runaway growth, not routine apt drift or minor upstream bumps. smoke-test.sh only — no base rebuild; affects the next tagged release's gate.