Persist authored Age generation plans (#31)

BUILDING.txt

@@ -74,9 +74,11 @@ Or, with mise:
 All required GameTests should pass before distributing a build.
 
 GameTests use `build/gametest-run/world`, separate from the normal development
-world. Before each run, Gradle removes that test world if it exceeds 512 MiB.
-Override the limit with `-PgameTestMaxWorldMiB=NUMBER`. A caller-supplied
-`-PgameTestRunDir` is never removed automatically.
+world. Gradle starts each default run with a fresh world and removes it after
+a successful run. Failed worlds are retained under
+`build/gametest-run/failed-worlds/` for inspection until the next successful
+run. Logs and configuration are kept. A caller-supplied `-PgameTestRunDir`
+is never removed automatically.
 
 To run the suite in a fresh temporary directory instead, use: