docs(#254): flesh out design doc + sync README image sizes
- Design doc 254: status Draft → Implemented; fill §4 (context),
§5 (per-layer impact), §6 (alternatives), §7 (env vars / build-args),
§8 (risks mapped to audit dims), §9 (testing + measured sizes),
§10 (rollout), §11 (deferred follow-ups), §12 (PR + Docling tools link).
- README image-variants table: latest-local 1.9 GB → 3.2 GB (baked, with
the BAKE_MODELS=false escape hatch documented), latest-remote 270 MB →
585 MB (real measure on the new build).