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/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,
         }
     }
 }