Decouple option codes, fix encode bugs, add test suite

Maintainability:
- Choice options now carry a stable `code` separate from the display
  `label`. The command builder and recipe storage key off codes, so
  relabeling a menu entry no longer silently changes the encode or
  breaks a saved recipe. Format/Audio dispatch is now a clean code
  match instead of `starts_with` on display strings. Old recipes that
  stored labels still load via a label fallback.

Correctness:
- Fade start times now scale by the speed factor already applied in the
  filter chain, so Speed + Fade produces a fade in the right place
  regardless of op order (video and audio).
- Two-pass pass 1 uses the platform null device (NUL on Windows).
- The stderr drain thread is joined before Finished is sent, so the
  failure tail reported to the user is never truncated.

Features / cleanup:
- Accept multiple file arguments on the CLI to open batch mode directly.
- try_recv() instead of recv_timeout(ZERO); drop unused import.

Testing:
- 23 unit tests covering build() arg generation, speed/fade ordering,
  recipe round-trip + old-label fallback, and the pure helpers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -204,7 +204,7 @@ checksum = "8f42a60cbdf9a97f5d2305f08a87dc4e09308d1276d28c869c684d7777685682"
 
 [[package]]
 name = "lazyff"
-version = "0.1.0"
+version = "0.2.0"
 dependencies = [
  "anyhow",
  "ratatui",