BREAKING: Figure.coast: Add parameter river_lakes for setting fills of river-lakes separately

[seisman]

This PR improves the alias of coast's -C option. The GMT CLI syntax is -Cfill[+l|+r].

Set the shade, color, or pattern for lakes and river-lakes [Default is the fill chosen for “wet” areas -S]. Optionally, specify separate fills by appending +l for lakes or +r for river-lakes, repeating the -C option as needed.

Previously, -C was aliased to lakes. This PR split the -C option into two parameters, lakes and river_lakes. Here is a comparison of old and new usages:

Description	Old	New
Set same fill for lakes and river-lakes	lakes="blue"	lakes="blue", river_lakes="blue"
Set fill for river-lakes only	lakes="blue+r"	river_lakes="blue"
Set fill for lakes only	lakes="blue+l"	lakes="blue"
Set separate fills for river-lakes and lakes	lakes=["blue+r", "green+l"]	lakes="green", river_lakes="blue"

Please note that in the old syntax, using lakes="blue" also set the fill color of river-lakes to "blue". If we preserved that behavior, there would be no Pythonic way to fill only lakes without also filling river-lakes. Therefore, I decided to alias lakes="blue" to -Cblue+l, meaning “fill lakes only.” This introduces a breaking change.

Related to #4240

[Copilot]

Pull request overview

This PR refactors Figure.coast’s handling of GMT’s -C option to provide a clearer long-form API for setting lake vs river-lake fills, while preserving backward compatibility with the existing lakes=[...] list syntax.

Changes:

• Added a dedicated _alias_option_C helper to build -C arguments from lakes/river_lakes, including a compatibility path for the legacy list form.
• Introduced new river_lakes parameter and updated coast validation logic and alias mapping accordingly.
• Updated docstring alias/parameter documentation around lake and river-lake fills.

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