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/indexer.ts

@@ -9,9 +9,9 @@
  * this module is the impure shell that spawns yt-dlp and writes to disk.
  */
 
-import { execFile } from 'child_process'
 import { existsSync } from 'fs'
 import { getYtdlpPath, YTDLP_MISSING_MSG } from './binaries'
+import { execFileAsync } from './lib/exec'
 import { INDEX_MAX_BUFFER, INDEX_TIMEOUT_MS } from './constants'
 import { cleanError } from './log'
 import { assertHttpUrl } from './url'
@@ -46,28 +46,24 @@ interface FlatInfo {
  * flat upload list can be a few MB of JSON; the timeout is generous for the same
  * reason. `--` terminates option parsing so the URL can't be read as a flag.
  */
-function probeFlat(url: string): Promise<FlatInfo> {
-  return new Promise((resolve, reject) => {
-    execFile(
-      getYtdlpPath(),
-      ['-J', '--flat-playlist', '--no-warnings', '--', url],
-      { windowsHide: true, maxBuffer: INDEX_MAX_BUFFER, timeout: INDEX_TIMEOUT_MS },
-      (err, stdout, stderr) => {
-        if (err) {
-          const msg = (err as { killed?: boolean }).killed
-            ? 'Timed out indexing source. Check the link or your connection.'
-            : cleanError(stderr) || err.message
-          reject(new Error(msg))
-          return
-        }
-        try {
-          resolve(JSON.parse(stdout) as FlatInfo)
-        } catch {
-          reject(new Error('Could not parse source info from yt-dlp.'))
-        }
-      }
+async function probeFlat(url: string): Promise<FlatInfo> {
+  const r = await execFileAsync(
+    getYtdlpPath(),
+    ['-J', '--flat-playlist', '--no-warnings', '--', url],
+    { maxBuffer: INDEX_MAX_BUFFER, timeout: INDEX_TIMEOUT_MS }
+  )
+  if (!r.ok) {
+    throw new Error(
+      r.timedOut
+        ? 'Timed out indexing source. Check the link or your connection.'
+        : cleanError(r.stderr) || r.error?.message || ''
     )
-  })
+  }
+  try {
+    return JSON.parse(r.stdout) as FlatInfo
+  } catch {
+    throw new Error('Could not parse source info from yt-dlp.')
+  }
 }
 
 /** Probe a channel tab (e.g. /videos, /playlists); never throws — returns null. */