Depth bands and isobaths from zoom 4

The style for the v2026.10.16 tiles.

• Bands mode and isobaths start at z4 instead of z6 (#214).
• The coarse depth ladders carry the 10 and 20 m contours, so shallow seas split into bands at low zoom (#214).
• BANDS_MIN_ZOOM is exported for apps that switch between Bands and Relief.