refactor(main): execFileAsync helper — unify the 5 spawn-and-read sites

Replace the hand-rolled `new Promise((resolve) => execFile(…, cb))` wrappers in
probe / ytdlp (×2) / ffmpeg / indexer.probeFlat / download.probeMeta with one
lib/exec.ts execFileAsync returning a normalized
{ok,stdout,stderr,timedOut,error} that never rejects. Each caller keeps its own
error mapping (timedOut -> "Timed out …" vs stderr) and JSON parsing; behaviour
is unchanged. windowsHide defaults on; per-call timeout/maxBuffer preserved.

Foundation for CC4/CC5 — the stdout line-buffer half lands with SIMP5, the
net.request half with SIMP16, at which point those checkboxes close.

Unit-tested (exec.test.ts) against the node binary: ok / stderr+nonzero-exit /
missing-binary / timeout. typecheck + 262 tests + eslint + prettier green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/download.ts

@@ -14,6 +14,7 @@ import {
   YTDLP_MISSING_MSG
 } from './binaries'
 import { getSettings } from './settings'
+import { execFileAsync } from './lib/exec'
 import { getDownloadArchivePath, getDefaultMediaDir } from './paths'
 import { ensureManagedYtdlp } from './ytdlp'
 import { materializeCookies, hasStoredCookies } from './cookies'
@@ -123,32 +124,28 @@ function logFailure(opts: StartDownloadOptions, title: string | undefined, error
 // itself contains a newline, since each --print field is emitted on its own line.
 const META_SEP = '\u001f'
 
-function probeMeta(ytdlp: string, url: string): Promise<DownloadMeta | null> {
-  return new Promise((resolve) => {
-    execFile(
-      ytdlp,
-      [
-        '--no-playlist',
-        '--no-warnings',
-        '--skip-download',
-        '--print',
-        `%(title)s${META_SEP}%(uploader)s${META_SEP}%(duration_string)s`,
-        '--',
-        url
-      ],
-      { windowsHide: true, maxBuffer: META_MAX_BUFFER, timeout: META_PROBE_TIMEOUT_MS },
-      (err, stdout) => {
-        if (err) return resolve(null)
-        const [title, uploader, duration] = stdout.split(META_SEP).map((l) => l.trim())
-        const clean = (v?: string): string | undefined => (v && v !== 'NA' ? v : undefined)
-        resolve({
-          title: clean(title),
-          channel: clean(uploader),
-          durationLabel: clean(duration)
-        })
-      }
-    )
-  })
+async function probeMeta(ytdlp: string, url: string): Promise<DownloadMeta | null> {
+  const r = await execFileAsync(
+    ytdlp,
+    [
+      '--no-playlist',
+      '--no-warnings',
+      '--skip-download',
+      '--print',
+      `%(title)s${META_SEP}%(uploader)s${META_SEP}%(duration_string)s`,
+      '--',
+      url
+    ],
+    { maxBuffer: META_MAX_BUFFER, timeout: META_PROBE_TIMEOUT_MS }
+  )
+  if (!r.ok) return null
+  const [title, uploader, duration] = r.stdout.split(META_SEP).map((l) => l.trim())
+  const clean = (v?: string): string | undefined => (v && v !== 'NA' ? v : undefined)
+  return {
+    title: clean(title),
+    channel: clean(uploader),
+    durationLabel: clean(duration)
+  }
 }
 
 // --- Argv construction (shared by startDownload and the command preview) ---