#886: say why footage the decoders can't read shows no picture

Problem: a movie whose profile the software decoders lack (H.264 High 4:2:2 / 10-bit)
probes fine, then every frame read fails; MediaPool logged the error per frame and returned
None, so the layer rendered black with no sign why in the viewer, Project panel or for agents.

Fix: MediaPool records the reason once per file (codec from the probe plus the decoder's text
without error wrappers; I/O errors stay with missing-footage handling), logs it once, and
exposes it through the new FootageSource::problem. The Composition viewer shows it on the
affected layer, the Project panel marks the item with a warning whose tooltip is the reason,
and project.summary / footage.info carry a "problem" field. Cleared when the file is
forgotten; only set when decoding really failed, so a hardware decoder that handles the file
shows nothing.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/render/src/lib.rs

@@ -85,6 +85,13 @@ pub trait FootageSource: Send + Sync {
     fn vector_frame(&self, _item: ItemId, _footage: &Footage, _scale: f64) -> Option<Arc<Image>> {
         None
     }
+    /// Why `footage` shows no picture although its file is there: a read of it failed because
+    /// no decoder handles its codec, profile, chroma format or bit depth, or the stream is damaged
+    /// (e.g. "H.264 (High 4:2:2) isn't supported by the built-in decoder: ..."). `None` while it
+    /// decodes, and for sources that don't track failures. Stays set until the file is forgotten.
+    fn problem(&self, _footage: &Footage) -> Option<String> {
+        None
+    }
 }
 
 /// Footage that can be rasterised at any scale (SVG).