Stage asset composer (Inkarnate-style map builder)

StageAsset model, 25 SVG sprites (nature/structures/hazards/markers), stage-asset-composer Stimulus controller, asset picker panel, placement overlay, ActionCable sync via StageChannel, 4 MCP tools (create/list/update/destroy_stage_asset, 145 tools total). ADR-0068. System tests: 5/5. Outer wrapper pattern required so Edit Map button (sticky header) is inside the Stimulus controller scope.