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/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 {