Refactor large Kotlin source files

- Split core game models from sandbox engine, native level setup, and repo state saving.

- Move app overlays and completion helpers out of GitHugApp.

- Extract the terminal special key bar into its own component.

app/src/main/java/solutions/tretter/githugandroid/CompletionCandidates.kt

@@ -0,0 +1,29 @@
+package solutions.tretter.githugandroid
+
+internal fun fileCompletionCandidates(repo: RepoState): List<String> {
+    val prefix = repo.currentDirPrefix()
+    return repo.files
+        .filterNot { it.deleted }
+        .map { it.name }
+        .filter { it.startsWith(prefix) }
+        .map { it.removePrefix(prefix) }
+        .filter { it.isNotBlank() }
+}
+
+internal fun directoryCompletionCandidates(repo: RepoState): List<String> {
+    val prefix = repo.currentDirPrefix()
+    return repo.files
+        .filterNot { it.deleted }
+        .map { it.name }
+        .filter { it.startsWith(prefix) }
+        .map { it.removePrefix(prefix) }
+        .flatMap { file ->
+            val parts = file.split('/').dropLast(1)
+            parts.indices.map { index -> parts.take(index + 1).joinToString("/") + "/" }
+        }
+        .distinct()
+}
+
+private fun RepoState.currentDirPrefix(): String {
+    return if (currentDir == ".") "" else currentDir.trimEnd('/') + "/"
+}