feat: todo app demo — streamable, showcases v2 builtins

Push, len, for-in with index, array access, if/then/else in elements,
string interpolation, input binding, streaming enabled.
40 lines of .ds code. Run: dreamstack stream examples/todo.ds

examples/todo.ds

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+-- DreamStack Todo App
+-- Showcases: push, len, for-in, string interpolation, array access, streaming
+-- Stream to phone: dreamstack stream examples/todo.ds --relay ws://YOUR_IP:9100
+
+let todos = ["Learn DreamStack", "Build something cool", "Ship it"]
+let done = [0, 0, 0]
+let input_text = ""
+let next_id = 3
+let filter_mode = "all"
+
+-- Derived counts
+let total = len(todos)
+
+-- Stream: any device on the network can view / interact
+stream todo on "ws://localhost:9100" { mode: signal }
+
+view app =
+  column [
+    text "DreamStack Todos"
+    text "{total} items"
+
+    -- Add new todo
+    row [
+      input "" { bind: input_text, placeholder: "What needs to be done?" }
+      button "Add" { click: push(todos, input_text) }
+    ]
+
+    -- Todo list
+    for item, i in todos ->
+      row [
+        button (if done[i] then "✅" else "⬜") { click: done[i] = (if done[i] then 0 else 1) }
+        text (if done[i] then "  ̶{item}̶" else item)
+      ]
+
+    -- Controls
+    row [
+      text "{total} items"
+      button "Clear All" { click: todos = [] }
+    ]
+  ]