fix(encode): order ffmpeg inputs ahead of output options (#143)

* fix(encode): order ffmpeg inputs ahead of output options

FFmpeg parses argv positionally: an option applies to the next `-i` that
follows it. The audio input was emitted after the codec block, so `-c:v`,
`-crf`, `-profile:v` and `-pix_fmt` were read as *input* options for
audio.raw and ffmpeg refused to start with "Option profile:v cannot be
applied to input url". Every scenario carrying an audio track — or an
embedded video with a soundtrack — failed to encode, on all four codecs,
through the default path.

Move the audio input next to the video input and keep `-c:a`/`-b:a` with
the output description. The argv assembly moves into `ffmpeg_args`, a pure
function, so the ordering invariant is unit-testable without an ffmpeg
binary on the machine.

A broken pipe here almost always means ffmpeg already died on its own
arguments, so `FfmpegWrite` now carries the tail of ffmpeg's stderr. It
used to be printed only outside `--quiet`, which left the diagnosis of
this very bug reading "Failed to write to FFmpeg pipe: Broken pipe".

* style(encode): apply rustfmt to the ffmpeg argv builder

Author: LeadcodeDev

crates/rustmotion-core/src/error.rs

@@ -161,8 +161,14 @@ pub enum RustmotionError {
     #[error("Failed to open FFmpeg stdin pipe")]
     FfmpegPipe,
 
-    #[error("Failed to write to FFmpeg pipe: {reason}")]
-    FfmpegWrite { reason: String },
+    // A broken pipe here nearly always means ffmpeg already died on its own
+    // arguments, so the useful diagnostic is ffmpeg's stderr rather than our
+    // write error. Carry it in the error so it survives `--quiet`.
+    #[error("Failed to write to FFmpeg pipe: {reason}{}", .stderr.as_ref().map(|s| format!("\nffmpeg reported:\n{}", s)).unwrap_or_default())]
+    FfmpegWrite {
+        reason: String,
+        stderr: Option<String>,
+    },
 
     #[error("Failed to wait for FFmpeg: {reason}")]
     FfmpegWait { reason: String },