feat(merge-split): add splitMergedFile() function using FFmpeg stream-copy

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

src/main.ts

@@ -2526,6 +2526,74 @@ async function mergeVideos(
     }
 }
 
+// ==========================================
+// SPLIT MERGED FILE
+// ==========================================
+async function splitMergedFile(
+    inputFile: string,
+    outputFolder: string,
+    partDurationSec: number,
+    totalDurationSec: number,
+    filenameGenerator: (partNum: number) => string,
+    onProgress: (currentPart: number, totalParts: number) => void
+): Promise<{ success: boolean; files: string[] }> {
+    const ffmpegReady = await ensureFfmpegInstalled();
+    if (!ffmpegReady) {
+        appendDebugLog('split-merged-missing-ffmpeg');
+        return { success: false, files: [] };
+    }
+
+    const ffmpeg = getFFmpegPath();
+    const numParts = Math.ceil(totalDurationSec / partDurationSec);
+    const splitFiles: string[] = [];
+
+    for (let i = 0; i < numParts; i++) {
+        if (currentDownloadCancelled) {
+            return { success: false, files: splitFiles };
+        }
+
+        const startSec = i * partDurationSec;
+        const thisDuration = Math.min(partDurationSec, totalDurationSec - startSec);
+        const outputFile = path.join(outputFolder, filenameGenerator(i + 1));
+
+        onProgress(i + 1, numParts);
+
+        const args = [
+            '-ss', formatDuration(startSec),
+            '-i', inputFile,
+            '-t', formatDuration(thisDuration),
+            '-c', 'copy',
+            '-y', outputFile
+        ];
+
+        appendDebugLog('split-merged-part', { part: i + 1, total: numParts, startSec, duration: thisDuration });
+
+        const success = await new Promise<boolean>((resolve) => {
+            const proc = spawn(ffmpeg, args, { windowsHide: true });
+            currentProcess = proc;
+
+            proc.on('close', (code) => {
+                currentProcess = null;
+                resolve(code === 0 && fs.existsSync(outputFile));
+            });
+
+            proc.on('error', () => {
+                currentProcess = null;
+                resolve(false);
+            });
+        });
+
+        if (!success) {
+            appendDebugLog('split-merged-part-failed', { part: i + 1, outputFile });
+            return { success: false, files: splitFiles };
+        }
+
+        splitFiles.push(outputFile);
+    }
+
+    return { success: true, files: splitFiles };
+}
+
 // ==========================================
 // DOWNLOAD FUNCTIONS
 // ==========================================