Run python-pptx code and return a preview image (public, no auth)

Executes python-pptx code in the sandbox and returns slide 1 as an inline PNG (768px) plus static lint findings (overlaps, off-canvas shapes, tiny fonts, chart semantics) in lint[]. Send a python-pptx script as it is (the presentation it saves is the one shown), or a build(prs) function. No .pptx is produced and nothing is downloadable — the file, PDF export, and full-resolution renders live in the playground and /v1/render/code. 16KB code cap, 10s timeout, 1 slide.