Test coverage gaps from the viewer rewrite #23

Known gaps in the tests added with #8 (behaviour is believed correct, but nothing would catch a regression):

• GLB import: rotation and scale composition are only tested with translations; an out-of-range scene index silently yields an empty document.
• Viewer engine: shading of the GLSL octahedral normal decode, RGBA8 pick fallback with slot ids ≥ 256, UV correctness in Textures mode, update-range targets of the state texture, mixed buckets (merged chunks and instanced cells sharing one material), shapes split into several cells, multi-chunk buckets in e2e, motion-detail integration in e2e.
• UI: highlight colour restore after deselect, Esc while an input has focus, Demolish/Temporary 4D tints, 4D speed mapping, tour stop by wheel, link matching by file name, the English empty state.
• Exporter: CLI error paths beyond the out-of-range index case; the streaming test name overclaims what it checks.