refactor: extract main runtime helper groups

- Extract remaining runtime helper clusters from main.ts into dedicated modules for readability:\n  - src/main/jlpt-runtime.ts\n  - src/main/media-runtime.ts\n  - src/main/overlay-visibility-runtime.ts\n- Wire main.ts to use the new runtime services and remove duplicated in-file helpers.\n- Preserve existing behavior via full typecheck + test:fast verification.\n- Finalize and archive TASK-56 backlog entry; update TASK-54 with completion metadata and summary.

src/main/media-runtime.ts

@@ -0,0 +1,70 @@
+import { updateCurrentMediaPathService } from "../core/services";
+
+import type { SubtitlePosition } from "../types";
+
+export interface MediaRuntimeDeps {
+  isRemoteMediaPath: (mediaPath: string) => boolean;
+  loadSubtitlePosition: () => SubtitlePosition | null;
+  getCurrentMediaPath: () => string | null;
+  getPendingSubtitlePosition: () => SubtitlePosition | null;
+  getSubtitlePositionsDir: () => string;
+  setCurrentMediaPath: (mediaPath: string | null) => void;
+  clearPendingSubtitlePosition: () => void;
+  setSubtitlePosition: (position: SubtitlePosition | null) => void;
+  broadcastSubtitlePosition: (position: SubtitlePosition | null) => void;
+  getCurrentMediaTitle: () => string | null;
+  setCurrentMediaTitle: (title: string | null) => void;
+}
+
+export interface MediaRuntimeService {
+  updateCurrentMediaPath: (mediaPath: unknown) => void;
+  updateCurrentMediaTitle: (mediaTitle: unknown) => void;
+  resolveMediaPathForJimaku: (mediaPath: string | null) => string | null;
+}
+
+export function createMediaRuntimeService(
+  deps: MediaRuntimeDeps,
+): MediaRuntimeService {
+  return {
+    updateCurrentMediaPath(mediaPath: unknown): void {
+      if (typeof mediaPath !== "string" || !deps.isRemoteMediaPath(mediaPath)) {
+        deps.setCurrentMediaTitle(null);
+      }
+
+      updateCurrentMediaPathService({
+        mediaPath,
+        currentMediaPath: deps.getCurrentMediaPath(),
+        pendingSubtitlePosition: deps.getPendingSubtitlePosition(),
+        subtitlePositionsDir: deps.getSubtitlePositionsDir(),
+        loadSubtitlePosition: () => deps.loadSubtitlePosition(),
+        setCurrentMediaPath: (nextPath: string | null) => {
+          deps.setCurrentMediaPath(nextPath);
+        },
+        clearPendingSubtitlePosition: () => {
+          deps.clearPendingSubtitlePosition();
+        },
+        setSubtitlePosition: (position: SubtitlePosition | null) => {
+          deps.setSubtitlePosition(position);
+        },
+        broadcastSubtitlePosition: (position: SubtitlePosition | null) => {
+          deps.broadcastSubtitlePosition(position);
+        },
+      });
+    },
+
+    updateCurrentMediaTitle(mediaTitle: unknown): void {
+      if (typeof mediaTitle === "string") {
+        const sanitized = mediaTitle.trim();
+        deps.setCurrentMediaTitle(sanitized.length > 0 ? sanitized : null);
+        return;
+      }
+      deps.setCurrentMediaTitle(null);
+    },
+
+    resolveMediaPathForJimaku(mediaPath: string | null): string | null {
+      return mediaPath && deps.isRemoteMediaPath(mediaPath) && deps.getCurrentMediaTitle()
+        ? deps.getCurrentMediaTitle()
+        : mediaPath;
+    },
+  };
+}