Add marshland in editor

[dvincent56]

Adds a new "Marshland" land tileset, selectable via the editor's Water submenu (Water / Marshland).

Climate-aware with seamless autotiling that blends into surrounding terrain and water (Central/Northern/Desert variants), including camera-rotation support.
Impassable to all walkers (citizens, traders, enemies, animals); permanent (cannot be cleared); blocks bridges (cannot build over or onto marshland).
Emits a negative desirability radius and a small health malus to nearby houses.
Distinct minimap colour (in-game, editor, and map-load preview) and a dedicated right-click info panel ("Marshland", a disease-breeding, ill-reputed swamp).
Editor scenarios (.mapx) now store terrain as u32 so the new flag persists on save.

Open points

• Marsh tiles straddling a coastline (land + water) aren't rendered correctly yet.
• Bridges can't cross marshland — to revisit later if low bridges over marsh are wanted.

[dvincent56]

New version: Marshland editor terrain (overlay-rendered)

Drawn as an overlay over the native terrain via a dedicated marsh image grid
instead of replacing the tile, so the base land/water/coast keeps its own
rendering and shows through the marsh's transparent edges. Impassable, lowers
desirability and house health nearby, blocks bridges, not bulldozable.

[Filu2259]

The health impact could create interesting scenarios.
What if buildings located near marshlands would distribute sickness points through their walkers in the same way as cartpushers (including cosmetic plebs?)?
Or other way to enhance epidemic potential?