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);

src/interpreter/types.rs

@@ -167,6 +167,7 @@ pub struct Element<'a> {
     pub children:       Vec<Element<'a>>,
     pub computed_style: ComputedStyle,
     pub element_id:     ElementId,
+    pub content_hash:   u64,
 }
 
 impl<'a> Element<'a> {
@@ -183,6 +184,7 @@ impl<'a> Element<'a> {
             children:       Vec::with_capacity(4),
             computed_style: ComputedStyle::default(),
             element_id:     ElementId(u32::MAX),
+            content_hash:   0,
         }
     }
 
@@ -193,6 +195,7 @@ impl<'a> Element<'a> {
             children:       Vec::with_capacity(4),
             computed_style: ComputedStyle::default(),
             element_id:     id,
+            content_hash:   0,
         }
     }
 }

src/renderer.rs

@@ -9,8 +9,25 @@ use iced::widget::{
 use iced::{Alignment, Background, Border, Length, Theme};
 use iced::font::Weight;
 use std::borrow::Cow;
+use std::cell::RefCell;
 use std::collections::HashMap;
 
+thread_local! {
+    static WIDGET_ID_CACHE: RefCell<HashMap<(u32, &'static str), iced::widget::Id>> = RefCell::new(HashMap::new());
+}
+
+fn get_or_create_widget_id(key: u32, prefix: &'static str) -> iced::widget::Id {
+    WIDGET_ID_CACHE.with(|cache| {
+        cache.borrow_mut()
+            .entry((key, prefix))
+            .or_insert_with(|| {
+                let s = Box::leak(format!("{prefix}:{key}").into_boxed_str());
+                iced::widget::Id::new(s)
+            })
+            .clone()
+    })
+}
+
 fn extract_var_binding(el: &Element, prop: &str) -> Option<String> {
     el.properties.iter()
         .find_map(|(k, v)| {
@@ -378,7 +395,8 @@ fn apply_universal_box_model<'a>(
         }
 
         if let Some(sid) = scrollable_id {
-            s = s.on_scroll(move |viewport| Message::ScrollableScrolled(sid, viewport.absolute_offset().y));
+            s = s.id(get_or_create_widget_id(sid as u32, "sc"))
+                .on_scroll(move |viewport| Message::ScrollableScrolled(sid, viewport.absolute_offset().y));
         }
 
         s.into()
@@ -1167,6 +1185,7 @@ fn render_input<'a>(
 
     let mut input = text_input(&placeholder, &value)
         .on_input(move |v| Message::InputChanged(var_name.clone(), v))
+        .id(get_or_create_widget_id(el.element_id.0, "ti"))
         .padding(padding);
 
     if let Some(w) = cs.width {