installer: move yt-dlp/ffmpeg under .mc_custom/installer/, clean platform-cache

- yt-dlp.exe, ffmpeg.exe now live in %appdata%/.mc_custom/installer/ so
  the .mc_custom root stays a clean Minecraft game folder. Existing
  binaries at the old location are migrated on first run.
- After a successful install, the platform-cache (downloaded fabric /
  forge / neoforge installer jars) is deleted — it's regenerable and
  was just wasting disk space.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/installer-rp/ytdlp.ts

@@ -3,17 +3,38 @@ import { promises as fs, createWriteStream, constants as fsConst } from 'node:fs
 import path from 'node:path'
 import https from 'node:https'
 import http from 'node:http'
-import { getMcCustomDir } from '../shared/paths.js'
+import { getMcCustomDir, getMcCustomInstallerDir } from '../shared/paths.js'
 import { loadComponentI18n } from '../shared/i18n.js'
 
 const { t } = loadComponentI18n('installer-rp')
 
 /**
  * 리소스팩 간편설치기는 Windows .exe 로 배포되므로 yt-dlp.exe 한 종류만 사용.
- * 경로: %appdata%/.mc_custom/yt-dlp.exe
+ * 경로: %appdata%/.mc_custom/installer/yt-dlp.exe
  */
 export function getYtDlpExePath(): string {
-  return path.join(getMcCustomDir(), 'yt-dlp.exe')
+  return path.join(getMcCustomInstallerDir(), 'yt-dlp.exe')
+}
+
+/**
+ * 0.2.1 이전 버전이 `.mc_custom/yt-dlp.exe` 에 받아둔 파일이 있으면 새 위치로
+ * 옮긴다. 마인크래프트 게임 폴더 루트가 외부 도구 파일로 더럽혀지지 않도록.
+ */
+async function migrateLegacyExe(target: string): Promise<void> {
+  const legacy = path.join(getMcCustomDir(), 'yt-dlp.exe')
+  if (legacy === target) return
+  try {
+    await fs.access(legacy, fsConst.F_OK)
+  } catch {
+    return
+  }
+  try {
+    await fs.mkdir(path.dirname(target), { recursive: true })
+    await fs.rename(legacy, target)
+  } catch {
+    // 권한·드라이브 문제 등으로 실패하면 그냥 새로 받으면 되므로 무시.
+    try { await fs.unlink(legacy) } catch { /* noop */ }
+  }
 }
 
 const YT_DLP_DOWNLOAD_URL =
@@ -29,6 +50,7 @@ export async function ensureYtDlpExe(
   log?: (line: string) => void
 ): Promise<string> {
   const target = getYtDlpExePath()
+  await migrateLegacyExe(target)
   if (await canExecute(target)) {
     log?.(t('log.ytdlpExists', { path: target }))
     return target