fix: forget the partition record with the keys that describe it

`forget_cached_keys` is the one place that means "this slot holds nothing
you can trust", and it left the partition record behind. A reused slot
rebuilds regardless — its content id changed, so the arena takes the build
path and the record is overwritten — but that made the safety an accident
of another code path rather than something this function guarantees.

Co-Authored-By: Claude Opus 5 <[email protected]>

src/scene/pipeline/mod.rs

@@ -4987,6 +4987,12 @@ impl Pipeline {
         self.silhouette_key = (usize::MAX, u64::MAX, [u32::MAX; 3], false);
         self.silhouette_source_key = (usize::MAX, usize::MAX, u64::MAX);
         self.render_sig = u64::MAX;
+        // The partition record describes uploads this slot is no longer
+        // entitled to reuse. A reused slot rebuilds anyway, because its content
+        // id changed, but leaving a stale record here would make that an
+        // accident rather than a guarantee.
+        self.partition_contributors.clear();
+        self.partition_depth_generation = u64::MAX;
     }
 
     /// Catch errors delivered after the previous upload/submit completed.