tabs: avoid two HashSet allocations in tab-change detection

Replace the symmetric_difference over two freshly-materialized HashSets
with direct contains_key lookups on the existing key sets. Semantically
identical (set difference non-empty iff any key is in exactly one set) but
removes two heap HashSet allocations plus hashing of every key on each
widget update. Also drops the now-unused HashSet import.

Implements compact-idiomatic-code-20 from the cosmic-comp optimization
report. Created with the help of DeepSeek V4 Flash.

src/shell/element/stack/tabs.rs

@@ -31,7 +31,7 @@ use keyframe::{
     functions::{EaseInOutCubic, EaseOutCubic},
 };
 use std::{
-    collections::{HashMap, HashSet, VecDeque},
+    collections::{HashMap, VecDeque},
     time::{Duration, Instant},
 };
 
@@ -757,12 +757,10 @@ where
             state.last_state = Some(current_state.clone());
         }
         let last_state = state.last_state.as_mut().unwrap();
-        let unknown_keys = current_state
+        let unknown_keys = current_state.len() != last_state.len()
-            .keys()
+            || current_state
-            .collect::<HashSet<_>>()
+                .keys()
-            .symmetric_difference(&last_state.keys().collect::<HashSet<_>>())
+                .any(|key| !last_state.contains_key(key));
-            .next()
-            .is_some();
 
         enum Difference {
             NewOrRemoved,