fix(encode): exempt --frame from the codec/container guard

crates/rustmotion-cli/src/commands/render.rs

@@ -56,7 +56,10 @@ pub fn cmd_render(
     // Refuse a codec the container cannot hold before rendering anything:
     // ffmpeg otherwise discovers it after every frame is done, and reports it
     // as a raw -22 with no output file.
-    {
+    //
+    // `--frame` is exempt: it writes a PNG still through `render_single_frame`
+    // and never reaches an encoder, so the codec is not part of that operation.
+    if frame.is_none() {
         let container = format
             .as_deref()
             .unwrap_or_else(|| output.extension().and_then(|e| e.to_str()).unwrap_or("mp4"));
@@ -262,10 +265,9 @@ pub fn cmd_watch(
     use notify::{RecursiveMode, Watcher};
     use std::sync::mpsc;
 
-    // Refuse a codec the container cannot hold before rendering anything:
-    // ffmpeg otherwise discovers it after every frame is done, and reports it
-    // as a raw -22 with no output file.
-    {
+    // Refuse a codec the container cannot hold before rendering anything (see
+    // `cmd_render`; `--frame` is exempt for the same reason).
+    if frame.is_none() {
         let container = format
             .as_deref()
             .unwrap_or_else(|| output.extension().and_then(|e| e.to_str()).unwrap_or("mp4"));

crates/rustmotion/src/encode/video/formats.rs

@@ -637,6 +637,18 @@ mod codec_container_tests {
         check_codec_container("av1", "mp4").expect("unknown codec must not be second-guessed");
     }
 
+    /// A `--frame` still is written by `render_single_frame`, never by an
+    /// encoder, so the codec is not part of that operation at all. The guard
+    /// is skipped for it in `cmd_render`/`cmd_watch`; this pins the reason a
+    /// `.png` extension is not in any codec's list.
+    #[test]
+    fn png_is_not_a_container_any_codec_claims() {
+        assert!(
+            check_codec_container("h264", "png").is_err(),
+            "a still is not an h264 container — the caller must not ask"
+        );
+    }
+
     /// These containers never reach the ffmpeg muxer.
     #[test]
     fn own_path_containers_are_left_alone() {