Third pillar (UI framework) design captured from session:
- component = supervised actor; render() is a pure projection of state
- reactivity = the actor loop (message -> with mut -> re-render),
  which is Svelte 4's assignment -> recompute; stay Svelte 4, not runes
- state / reactive / handler regions; `reactive` is a checked manifest
  label (pure + zero-arg + reads state), cross-checked by the compiler
- `derive`/`derived` rejected: `derive` reserved (principles.md:250),
  `derived` a dyslexia near-homograph
- no VDOM (compile-time surgical updates); effects stay explicit and
  message-triggered so the UI is replayable

@@ -0,0 +2,4 @@
+
+**Status:** proposed 2026-07-07 (design captured from session). The *model* below is decided; surface-syntax items marked **provisional** are open and flagged in [Provisional syntax](#provisional-syntax).
+
+**One line:** a UI component is an actor; its reactivity is the actor loop (`message → new state → re-render`), which is exactly Svelte 4's `assignment → recompute` with two principled deviations — the mutation trigger is `with mut` (not `=`), and derived values carry a **checked, manifest label** (`reactive`) rather than an inferred sigil (`$:`) or a per-value rune (`$derived()`).

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+`actor Main` and has **no `fn main`** of its own. It supplies the `fn main`:
+
+```
+fn main() -> Unit {