EPD Collect Artifacts

Reads epd-products.json and epd-candidates.json (already EPD-filtered). Tier order per product: a verified vault document is copied (never re-verified); else up to 3 manufacturer-EPD candidates are tried in rank order until one downloads and verifies; else a product carbon footprint/LCA is verified against both product name and A1-A3/cradle-to-gate boundary; else an industrial/sector-average EPD is applied as proxy, trying any vault category document first before the registry hits. Downloads the PDF to /workspace/work/outputs/epds// and writes /workspace/work/code/epd-download.json. Never invents EPD metadata or PDF bytes. Library workbook is produced by epd-build-library.