bucket_catch: make the done phase carry its result

Removes the last fixable non-null assertion. GamePhase goes from a string
union to a tagged one whose "done" arm carries `result: GameResult`, so
narrowing on phase.kind yields a non-null result and `fileResult!`
disappears. GameResult - previously an untagged union nothing consumed -
is now tagged, so the render picks a screen from the discriminant instead
of inferring it from which of two nullable states was set.

Three earlier attempts at this site were reverted: a null guard returning
null, a widened ReactElement | null return, and a tagged GameResult with
phase left as a string. Each either added a branch no test could reach -
dropping the 100% gate - or left the value nullable in the fall-through.
The invariant lived in the phase, so the fix had to as well.

Coverage stays 100%. Statements 585 -> 579 and lines 536 -> 530, exactly
the two useState declarations and their setters that the collapse
removed; branches unchanged at 217, so nothing new went untested. All 10
App tests pass unmodified - they drive the app through mocked onDone
callbacks, so both done arms are exercised through the new union.

eslint 5 -> 4 (45 -> 4 for the session). The remaining four are reported
in the previous commit's evidence. usePuzzleGameLoop's Map.get assertion
is not among the fixable ones: `?? Infinity` was tried and measured at
219 branches / 99.54%, since the default side is unreachable.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011w2Pj4Ek3ZNMQVYcFHgXDY

bucket_catch/packages/frontend/src/App.tsx

@@ -13,55 +13,52 @@ import { ScoreScreen } from "./components/ScoreScreen";
 import { PuzzleResult } from "./components/PuzzleResult";
 
 export default function App(): React.ReactElement {
-  const [phase, setPhase] = useState<GamePhase>("drop");
+  const [phase, setPhase] = useState<GamePhase>({ kind: "drop" });
   const [files, setFiles] = useState<File[]>([]);
   const [mode, setMode] = useState<TransferMode>("download");
   const [puzzleGridSize, setPuzzleGridSize] = useState(4);
-  const [fileResult, setFileResult] = useState<FileGameResult | null>(null);
-  const [puzzleResult, setPuzzleResult] = useState<PuzzleGameResult | null>(null);
 
   const handleFiles = useCallback((incoming: File[]) => {
     setFiles(incoming);
-    setPhase("mode");
+    setPhase({ kind: "mode" });
   }, []);
 
-  const handlePuzzleDirect = useCallback((imageFile: File, gridSize: number) => {
+  const handlePuzzleDirect = useCallback(
+    (imageFile: File, gridSize: number) => {
       setFiles([imageFile]);
       setMode("puzzle");
       setPuzzleGridSize(gridSize);
-    setPhase("playing");
-  }, []);
+      setPhase({ kind: "playing" });
+    },
+    [],
+  );
 
   const handleStart = useCallback((selected: TransferMode, gridSize = 4) => {
     setMode(selected);
     if (selected === "puzzle") setPuzzleGridSize(gridSize);
-    setPhase("playing");
+    setPhase({ kind: "playing" });
   }, []);
 
-  const handleFileDone = useCallback((result: FileGameResult) => {
-    setFileResult(result);
-    setPhase("done");
+  const handleFileDone = useCallback((value: FileGameResult) => {
+    setPhase({ kind: "done", result: { kind: "file", value } });
   }, []);
 
-  const handlePuzzleDone = useCallback((result: PuzzleGameResult) => {
-    setPuzzleResult(result);
-    setPhase("done");
+  const handlePuzzleDone = useCallback((value: PuzzleGameResult) => {
+    setPhase({ kind: "done", result: { kind: "puzzle", value } });
   }, []);
 
   const handleRestart = useCallback(() => {
     setFiles([]);
-    setFileResult(null);
-    setPuzzleResult(null);
-    setPhase("drop");
+    setPhase({ kind: "drop" });
   }, []);
 
-  if (phase === "drop") {
+  if (phase.kind === "drop") {
     return <DropZone onFiles={handleFiles} onPuzzle={handlePuzzleDirect} />;
   }
-  if (phase === "mode") {
+  if (phase.kind === "mode") {
     return <ModeSelect files={files} onStart={handleStart} />;
   }
-  if (phase === "playing") {
+  if (phase.kind === "playing") {
     if (mode === "puzzle") {
       return (
         <PuzzleCanvas
@@ -73,13 +70,15 @@ export default function App(): React.ReactElement {
     }
     return <GameCanvas files={files} onDone={handleFileDone} />;
   }
-  // done phase — puzzleResult is non-null iff mode was "puzzle"
-  if (puzzleResult !== null) {
-    return <PuzzleResult result={puzzleResult} onRestart={handleRestart} />;
+  // done phase: the phase carries its result, so both arms are non-null.
+  if (phase.result.kind === "puzzle") {
+    return (
+      <PuzzleResult result={phase.result.value} onRestart={handleRestart} />
+    );
   }
   return (
     <ScoreScreen
-      result={fileResult!}
+      result={phase.result.value}
       mode={mode}
       onRestart={handleRestart}
     />

bucket_catch/packages/frontend/src/types.ts

@@ -1,4 +1,12 @@
-export type GamePhase = "drop" | "mode" | "playing" | "done";
+/**
+ * Where the app is. The done phase carries its result, so "finished with no
+ * result" cannot be represented and the render never has to assert one away.
+ */
+export type GamePhase =
+  | { kind: "drop" }
+  | { kind: "mode" }
+  | { kind: "playing" }
+  | { kind: "done"; result: GameResult };
 
 export type TransferMode = "download" | "upload" | "puzzle";
 
@@ -52,4 +60,7 @@ export interface PuzzleGameResult {
   gridSize: number;
 }
 
-export type GameResult = FileGameResult | PuzzleGameResult;
+/** The outcome of a finished game, tagged by which mode produced it. */
+export type GameResult =
+  | { kind: "file"; value: FileGameResult }
+  | { kind: "puzzle"; value: PuzzleGameResult };