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/lib/exec.ts

@@ -0,0 +1,52 @@
+import { execFile, type ExecFileOptions } from 'child_process'
+
+/**
+ * Normalized result of a one-shot child-process run. `ok` is true on a clean
+ * exit-0; `timedOut` distinguishes a kill-by-timeout (Node sets `err.killed` when
+ * it terminates the process on the `timeout` option) from an ordinary non-zero
+ * exit, so callers can show "Timed out …" versus the captured stderr.
+ */
+export interface ExecResult {
+  ok: boolean
+  stdout: string
+  stderr: string
+  timedOut: boolean
+  /** The raw error on a non-zero exit / spawn failure (undefined when ok). */
+  error?: Error
+}
+
+/**
+ * `execFile` as a promise that never rejects (audit SIMP2/CC4/CC5). Replaces the
+ * six hand-rolled `new Promise((resolve) => execFile(…, cb))` wrappers across
+ * probe / ytdlp (×2) / ffmpeg / indexer / download.probeMeta / schedule — each
+ * caller now maps this normalized result to its own shape instead of repeating the
+ * `err.killed` timeout check and the resolve/reject plumbing.
+ *
+ * `windowsHide` defaults on (every caller wants the console window suppressed);
+ * pass `timeout` / `maxBuffer` per call exactly as before.
+ */
+export function execFileAsync(
+  file: string,
+  args: readonly string[],
+  options: ExecFileOptions = {}
+): Promise<ExecResult> {
+  return new Promise((resolve) => {
+    execFile(file, [...args], { windowsHide: true, ...options }, (err, stdout, stderr) => {
+      // With the default (utf8) encoding stdout/stderr are strings, but ExecFileOptions
+      // permits a Buffer encoding, so normalize defensively.
+      const out = typeof stdout === 'string' ? stdout : stdout.toString()
+      const errOut = typeof stderr === 'string' ? stderr : stderr.toString()
+      if (err) {
+        resolve({
+          ok: false,
+          stdout: out,
+          stderr: errOut,
+          timedOut: (err as { killed?: boolean }).killed === true,
+          error: err
+        })
+      } else {
+        resolve({ ok: true, stdout: out, stderr: errOut, timedOut: false })
+      }
+    })
+  })
+}