SEKO-03: Wire H7 Low-Value Declaration into Demo Flow (retry 1)

EU H7 declaration (eu_h7.py) is built and exposed via MCP but NOT called in the demo. The de minimis reform (EUR 150 → removal in July 2028) is a key SEKO selling point — show we handle both H1 (full) and H7 (simplified) paths. WHAT: Add H7 declaration output to demo_seko_triangle.py for the Ceramic Mug (value < EUR 150): 1. After FR duty calc, call build_h7_declaration() for low-value items 2. Show H7 data elements in demo output (simplified vs H1 full declaration) 3. Add a "De Minimis Reform Ready" badge/note showing C8 handles the transition 4. TSX component should show H7 vs H1 indicator on FR card FILES: - c8_platform/demo_seko_triangle.py — add H7 call for low-value products - c8_platform/demo_website/seko-triangle-demo.tsx — H7/H1 indicator on FR card - c8_platform/rosetta_core/eu_h7.py — verify it works with demo product data ACCEPTANCE: Demo output shows H7 path for Ceramic Mug (low value), H1 path for Steel I-Beam (high value), with de minimis reform callout.