Phase 8: StyleCache memoization

Add StyleCache struct with hash-keyed LRU-eviction strategy and
integrate into StyleSheet via Mutex<StyleCache> so no signature
changes are needed on evaluate_vdom_incr.

- StyleCache::get_or_compute hashes (type_name, props, epoch) -> ComputedStyle
- LRU: full clear on overflow (max 1024)
- RefCell avoided in favor of Mutex so StyleSheet remains Sync
  for parallel matching_rules_batch
- Manual Clone impl skips cache (fresh cache on clone)
- Call sites in evaluate_vdom_incr switched from ComputedStyle::compute
  to stylesheet.compute_cached
- Cache cleared in build_index after epoch increment (stale entries)

src/interpreter/style.rs

@@ -1,5 +1,9 @@
 use std::collections::{HashMap, HashSet};
 use std::borrow::Cow;
+use std::sync::Mutex;
+use std::hash::{Hash, Hasher};
+
+use super::reactive::ElementId;
 
 
 #[derive(Debug, Clone)]
@@ -458,10 +462,70 @@ impl StyleRule {
 
 
 #[derive(Debug, Clone)]
+pub struct StyleCache {
+    entries: HashMap<u64, ComputedStyle>,
+    max_entries: usize,
+}
+
+impl StyleCache {
+    pub fn new(max_entries: usize) -> Self {
+        Self { entries: HashMap::new(), max_entries }
+    }
+
+    pub fn get_or_compute(
+        &mut self,
+        type_name: &str,
+        props: &[(Cow<'_, str>, Cow<'_, str>)],
+        epoch: u64,
+        matched_sheets: &[&HashMap<String, String>],
+    ) -> ComputedStyle {
+        let key = {
+            let mut hasher = std::collections::hash_map::DefaultHasher::new();
+            type_name.hash(&mut hasher);
+            for (k, v) in props {
+                k.hash(&mut hasher);
+                v.hash(&mut hasher);
+            }
+            epoch.hash(&mut hasher);
+            hasher.finish()
+        };
+
+        if let Some(cached) = self.entries.get(&key) {
+            return cached.clone();
+        }
+
+        let style = ComputedStyle::compute(props, matched_sheets);
+
+        if self.entries.len() >= self.max_entries {
+            self.entries.clear();
+        }
+        self.entries.insert(key, style.clone());
+
+        style
+    }
+
+    pub fn clear(&mut self) {
+        self.entries.clear();
+    }
+}
+
+#[derive(Debug)]
 pub struct StyleSheet {
     rules: Vec<StyleRule>,
     index: Option<StyleIndex>,
     epoch: u64,
+    cache: Mutex<StyleCache>,
+}
+
+impl Clone for StyleSheet {
+    fn clone(&self) -> Self {
+        Self {
+            rules: self.rules.clone(),
+            index: self.index.clone(),
+            epoch: self.epoch,
+            cache: Mutex::new(StyleCache::new(1024)),
+        }
+    }
 }
 
 impl Default for StyleSheet {
@@ -470,6 +534,7 @@ impl Default for StyleSheet {
             rules: Vec::new(),
             index: None,
             epoch: 0,
+            cache: Mutex::new(StyleCache::new(1024)),
         }
     }
 }
@@ -488,6 +553,7 @@ impl StyleSheet {
 
     pub fn build_index(&mut self) {
         self.epoch += 1;
+        self.cache.lock().unwrap().clear();
         let mut index = StyleIndex::new();
         index.epoch = self.epoch;
 
@@ -749,6 +815,20 @@ impl StyleSheet {
         props
     }
 
+    pub fn compute_cached<'a>(
+        &self,
+        type_name: &str,
+        props: &[(Cow<'_, str>, Cow<'_, str>)],
+        matched_sheets: &[&'a HashMap<String, String>],
+    ) -> ComputedStyle {
+        let epoch = self.epoch;
+        self.cache.lock().unwrap().get_or_compute(type_name, props, epoch, matched_sheets)
+    }
+
+    pub fn clear_cache(&self) {
+        self.cache.lock().unwrap().clear();
+    }
+
     pub fn is_empty(&self) -> bool {
         self.rules.is_empty()
     }