Add optional export filename, tests for post-implement features

Add optional filename field to export dialog with automatic .json
extension handling. Extract resolveFilename() for testability. Add
tests for includeHistory flag, bundleToJson, and filename resolution.
Add export format compatibility note to CLAUDE.md.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/src/components/action-bar.tsx

@@ -459,14 +459,14 @@ export function ActionBar({
 	>(null);
 
 	const handleExportDownload = useCallback(
-		(includeHistory: boolean) => {
+		(includeHistory: boolean, filename: string) => {
 			const bundle = assembleExportBundle(
 				encounter,
 				undoRedoState,
 				playerCharacters,
 				includeHistory,
 			);
-			triggerDownload(bundle);
+			triggerDownload(bundle, filename);
 		},
 		[encounter, undoRedoState, playerCharacters],
 	);