test: add automated latitude validation via GameTest server

Run `./gradlew runGameTestServer` to validate the latitude biome system
without manual in-game testing. Boots a headless game server (full biome
registry), samples a 32000x32000 grid through LatitudeBiomeSource, renders a
PNG map and writes a distribution report, then asserts climate invariants:
  - spawn on a safe biome (plains/forest)
  - FROZEN band dominated by cold/frozen biomes (>60%)
  - HOT band dominated by warm/hot biomes (>60%)
  - swamp remains rare in the temperate band (<15%)

Outputs land in run/gametest-results/latitude/ (latitude_map.png +
latitude_report.txt). Sampling is BiomeSource-only (no chunk generation),
so it runs in ~35s regardless of installed mods.

- LatitudeGameTest: @GameTestHolder + @PrefixGameTestTemplate(false)
- structure/empty_1x1.nbt: minimal 1x1x1 air structure required by GameTest
- build.gradle: add gameTestServer run configuration

build.gradle

@@ -52,6 +52,11 @@ neoForge {
             systemProperty 'neoforge.enabledGameTestNamespaces', project.mod_id
         }
 
+        gameTestServer {
+            type = 'gameTestServer'
+            systemProperty 'neoforge.enabledGameTestNamespaces', project.mod_id
+        }
+
         data {
             data()
             programArguments.addAll '--mod', project.mod_id, '--all', '--output', file('src/generated/resources/').getAbsolutePath(), '--existing', file('src/main/resources/').getAbsolutePath()