Stage full-canvas viewport

stage layout (no sidebar), stage-viewport Stimulus controller (pan/zoom, pointer capture, wheel zoom, Ctrl+0 fit-to-viewport, panel toggle), HUD overlay pattern (data-hud attr prevents pan, data-no-pan for placement overlay). Canvas natural size cols×64px independent of DB grid_size. Panel toggle button at absolute left-full so it lands at x=0 when panel is closed. fitToViewport() initial fit on connect. transition-transform on panel. JS + CSS rebuild required on deploy. ADR-0070.