Phase 9: Keyed widgets (9.1 + 9.2)

9.1: Add content_hash field to Element, computed during VDOM evaluation
  from type_name + properties + children content_hashes via DefaultHasher.

9.2: Assign stable iced::widget::Id to text_input and scrollable widgets
  so Iced preserves widget state (cursor position, scroll offset) across
  frames. Ids are cached in a thread_local HashMap to avoid leaks.

9.3 skipped: iced::Element is not Clone and contains Box<dyn Widget>,
  making element-level caching impractical without unsafe lifetime hacks.

src/interpreter/mod.rs

@@ -7,6 +7,7 @@ pub mod style;
 pub mod types;
 
 use std::borrow::Cow;
+use std::hash::{Hash, Hasher};
 pub use rhei::RheiContext;
 use style::StyleSheet as SS;
 pub use types::{ComponentDef, Document, Element, Interner, InterpError};
@@ -476,6 +477,7 @@ impl Interpreter {
                         vcomp.children = Self::evaluate_vdom_incr(
                             &comp_child_refs, variables, components, rhei, stylesheet, &child_ancestors, dirty_set,
                         );
+                        vcomp.content_hash = Self::compute_content_hash(&vcomp);
                         output.push(vcomp);
 
                         for (k, old) in old_vals.into_iter().rev() {
@@ -509,6 +511,7 @@ impl Interpreter {
                         vnode.children = Self::evaluate_vdom_incr(
                             &child_refs, variables, components, rhei, stylesheet, &child_ancestors, dirty_set,
                         );
+                        vnode.content_hash = Self::compute_content_hash(&vnode);
                         output.push(vnode);
                     }
                 }
@@ -518,6 +521,20 @@ impl Interpreter {
         output
     }
 
+    fn compute_content_hash(el: &Element) -> u64 {
+        use std::collections::hash_map::DefaultHasher;
+        let mut h = DefaultHasher::new();
+        el.type_name.hash(&mut h);
+        for (k, v) in &el.properties {
+            k.hash(&mut h);
+            v.hash(&mut h);
+        }
+        for child in &el.children {
+            child.content_hash.hash(&mut h);
+        }
+        h.finish()
+    }
+
     fn build_ancestor_chain<'a>(ancestors: &[AncestorInfo], el: &Element) -> Vec<AncestorInfo> {
         let mut chain = ancestors.to_vec();
         let id = el.id().map(String::from);