Bundle ffprobe.exe and complete the real-download smoke-test pass

A live smoke test of buildArgs' argv against the bundled yt-dlp + ffmpeg found
that ffprobe.exe was never bundled (resources/bin/ shipped only ffmpeg.exe).
yt-dlp resolves both from --ffmpeg-location, so duration-aware post-processing
failed at runtime for every user with "ffprobe not found": --sponsorblock-remove,
--force-keyframes-at-cuts, and --split-chapters (CODE-AUDIT C1).

- Bundle ffprobe.exe (matching n8.1.2 LGPL build, sha256-verified against the
  already-bundled ffmpeg.exe) and document it in resources/bin/README.md as a
  required binary; correct the stale "not committed to git" note.
- Guard startDownload against a missing ffmpeg.exe/ffprobe.exe up front so the
  failure is a clear AeroFetch error, not a cryptic yt-dlp postprocessing one
  (new getFfprobePath() in binaries.ts).
- Expand test/real-download.integration.test.ts from 2 to 8 live cases: crop,
  sponsorblock-remove, audio opus re-encode, mkv+vp9 merge, subtitle+chapter
  embed, restrict-filenames, download-archive skip, and Phase C extra-args.
  All 8 pass against live yt-dlp + ffmpeg.
- ROADMAP: mark the Phase A/B/C smoke-test caveats done. CODE-AUDIT: add C1.

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

resources/bin/README.md

@@ -1,16 +1,18 @@
 # Bundled binaries
 
 AeroFetch spawns these from the **main process** and bundles them via
-electron-builder `extraResources` (they ship outside the asar archive). They are
-**not** committed to git (see `.gitignore`).
+electron-builder `extraResources` (they ship outside the asar archive). The
+`.exe` binaries are committed to the repo so a fresh clone builds and runs
+without a manual download step.
 
-Drop the two required Windows executables here before running `npm run dev` or
+Drop the three required Windows executables here before running `npm run dev` or
 building; `aria2c.exe` is optional:
 
 ```
 resources/bin/
 ├── yt-dlp.exe
 ├── ffmpeg.exe
+├── ffprobe.exe
 └── aria2c.exe   (optional)
 ```
 
@@ -20,13 +22,17 @@ resources/bin/
 - Grab `yt-dlp.exe`.
 - Unlicense / public domain.
 
-## ffmpeg.exe
+## ffmpeg.exe + ffprobe.exe
 
 - Use an **LGPL** build to keep licensing simple, e.g.
   https://github.com/BtbN/FFmpeg-Builds/releases (pick a `*-lgpl-shared` or
   `*-lgpl` win64 build) or https://www.gyan.dev/ffmpeg/builds/.
-- Copy `ffmpeg.exe` here. Ship the LGPL license text alongside the installer
-  before release.
+- Copy **both** `ffmpeg.exe` and `ffprobe.exe` here — they ship together in the
+  same archive's `bin/` folder, and their versions must match. yt-dlp needs
+  `ffprobe.exe` to read media durations; without it, duration-dependent
+  post-processing fails with "ffprobe not found" (`--sponsorblock-remove`,
+  `--force-keyframes-at-cuts`, and `--split-chapters` all error out).
+- Ship the LGPL license text alongside the installer before release.
 
 ## aria2c.exe (optional)