Remove small islands on compact maps with half threshold 🏝️ (#3762)

## Description:

Small-island removal previously ran only on the full-size terrain. The
compact (4x downscaled) map inherited that filtering before being
downscaled, which meant islands that survived at full size could end up
as tiny specks in the compact map.

This PR runs `removeSmallIslands` on the 4x terrain as well, with half
the threshold (15 vs 30 tiles), so very small islands are pruned in
compact maps while the full-size map is unchanged.

## Please complete the following:

- [X] I have added screenshots for all UI updates
- [X] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [X] I have added relevant tests to the test directory
- [X] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

FloPinguin

map-generator/map_generator.go

@@ -148,10 +148,11 @@ func GenerateMap(ctx context.Context, args GeneratorArgs) (MapResult, error) {
 		}
 	}
 
-	removeSmallIslands(ctx, terrain, args.RemoveSmall)
+	removeSmallIslands(ctx, terrain, minIslandSize, args.RemoveSmall)
 	processWater(ctx, terrain, args.RemoveSmall)
 
 	terrain4x := createMiniMap(terrain)
+	removeSmallIslands(ctx, terrain4x, minIslandSize/2, args.RemoveSmall)
 	processWater(ctx, terrain4x, false)
 
 	terrain16x := createMiniMap(terrain4x)
@@ -275,6 +276,7 @@ func processShore(ctx context.Context, terrain [][]Terrain) []Coord {
 		for y := 0; y < height; y++ {
 			tile := &terrain[x][y]
 			neighbors := getNeighbors(x, y, terrain)
+			tile.Shoreline = false
 
 			if tile.Type == Land {
 				// Land tile adjacent to water is shoreline
@@ -490,7 +492,8 @@ func getArea(x, y int, terrain [][]Terrain, visited map[string]bool) []Coord {
 
 // removeSmallIslands identifies and removes small land masses from the terrain.
 // If removeSmall is true, any removed bodies are converted to Water.
-func removeSmallIslands(ctx context.Context, terrain [][]Terrain, removeSmall bool) {
+// Land bodies smaller than minSize are removed.
+func removeSmallIslands(ctx context.Context, terrain [][]Terrain, minSize int, removeSmall bool) {
 	logger := LoggerFromContext(ctx)
 	if !removeSmall {
 		return
@@ -526,7 +529,7 @@ func removeSmallIslands(ctx context.Context, terrain [][]Terrain, removeSmall bo
 	smallIslands := 0
 
 	for _, body := range landBodies {
-		if body.size < minIslandSize {
+		if body.size < minSize {
 			logger.Debug(fmt.Sprintf("Removing small island at %d,%d (size %d)", body.coords[0].X, body.coords[0].Y, body.size), RemovalLogTag)
 			smallIslands++
 			for _, coord := range body.coords {
@@ -536,7 +539,7 @@ func removeSmallIslands(ctx context.Context, terrain [][]Terrain, removeSmall bo
 		}
 	}
 
-	logger.Info(fmt.Sprintf("Identified and removed %d islands smaller than %d tiles", smallIslands, minIslandSize))
+	logger.Info(fmt.Sprintf("Identified and removed %d islands smaller than %d tiles", smallIslands, minSize))
 }
 
 // packTerrain serializes the terrain grid into a byte slice.