renderCompleteDashboardAsSixel canvas

Chose one complete dashboard Sixel canvas over per-widget Sixel output or mixed ASCII/Sixel because a Sixel DCS advances the cursor and cannot coexist safely with the character framebuffer. On capable terminals with known cell geometry, rasterize every widget at its original grid coordinates. If any Sixel prerequisite or render step fails, always return the complete established character dashboard; partial or mixed fallback corrupts layout.