V100 COMPILE FIX FOR USGS-STYLE FINITE-FAULT EXPORT

Base
- Built from V99.

Fix
- V99 referenced grid_ocean_depth_m(), which does not exist in this codebase.
- Added finite_fault_export_ocean_depth_m() and changed the map-panel water coloring to use it.
- The helper reads g_grid.effective_depth_m when available, otherwise g_grid.depth_m.

No behavior changes except fixing the compile error in finite_fault_draw_map_panel().
