ACCESS-ESM: Distribute meltwater according to basal / iceberg fractions

[anton-seaice]

In the access-esm mediator code, we need to distribute ice-sheet meltwater according to two distinct patterns:

• For icebergs, spread meltwater using a climatology - the work in Rofi spread #13 and Calving flux via iceberg distribution map access-om3-configs#404 can be be refactored for this
• For basal melt, or melt directly from icesheets, spread the water around the coastlines. (The pattern of meltwater from JRA55do friver field could be used for this).

The volume of meltwater comes from the UM - this needs to have a configurable split for how much is allocated to iceberg vs basal melt. (Start at 50:50)

[anton-seaice]

There's a couple of useful links in https://forum.access-hive.org.au/t/cosima-twg-meeting-minutes-2026/5906/11