Render Queue: two outputs of one job never write the same file, and default names avoid the queue's other outputs

Two queue items that resolve to one output (the same comp added twice, a duplicated item) replaced
each other's file inside one job. Nothing is on disk when the job starts, so no check said anything.

- A default output name (renderQueue.add, render.preRender, comp.saveFrameAs {queue},
  file.createProxy) that another item of the queue writes, in any output module or segment, is
  numbered like a footage collision: Main.mp4, Main_1.mp4, Main_2.mp4. Sequences are compared by
  the frames they write, so two stills of one comp at different times keep their names. Names a
  caller gave and names already stored in items are never changed.
- Outputs of one job that still name the same file (explicit names, renderQueue.duplicate, output
  modules, segments) refuse the whole render before anything is written, naming both items.
  Sequences clash only on frames both write; a later item with Skip Existing Files leaves the
  earlier item's frames alone; items refused for footage take no part.

Refusing the whole render, not just the later item: which of two items should keep the file is the
user's to say, and failing only the later one would leave a half-rendered queue found after they
left. renderQueue.duplicate stays a verbatim copy so the clash is reported, not hidden.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RDvxCS8xcR28VArjzn1x7U

crates/engine/src/render_queue.rs

@@ -493,6 +493,11 @@ impl Session {
         let formats = exporter.formats();
         let (mut items, mut problems) = self.queued_outputs(&formats);
         let refused = self.check_outputs(&items);
+        // Two outputs of this job on one file: the render would replace its own output, so it is
+        // refused before anything is written (the items refused above take no part).
+        if let Some(refusal) = crate::output_guard::duplicate_refusal(&self.duplicate_outputs(&items, &refused)) {
+            return Err(refusal);
+        }
         items.retain(|modules| modules.first().is_none_or(|(it, _)| !refused.iter().any(|r| r.item == it.id)));
         problems.extend(refused.iter().map(|r| (r.item, RenderStatus::Failed(r.reason.clone()))));
         if !problems.is_empty() {