feat(lang): L11 views and templates - the UI is markup, not code

A `view Name { field = x; function q(..); on e(..) }` declaration is the one bridge between a
program and its UI: it writes view_<name>() -> UiView, whose model is a Value of every field and
whose call runs a query or an event by name.

ludic.ui is a template runtime:
- HTML-shaped XML screens and components, loaded at run time;
- {expression} bindings, if/else/each, props, slots, per-instance state;
- on-press / onclick actions (event, set, emit);
- component libraries (export="true", <import src as>).

Styling:
- stylesheets in <style> or importable .lss files (@import);
- CSS selectors (#id, .class, [attr=v], descendant and > combinators, :hover, :disabled,
  :first-child, :last-child, :nth-child, :not) weighed by specificity;
- the box model and flex under CSS's property names.

Also:
- default parameters, and positional-then-named calls;
- Value gains a float kind;
- a function shadowing a runtime one is refused;
- an index is evaluated before the slice is read;
- runtime errors name the right file.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.ui/screen.ludic

@@ -0,0 +1,17 @@
+# screen.ludic - the 2D screen as a renderer, for a program that draws with Screen.*
+function sc_rect(x: float, y: float, w: float, h: float, c: int, a: float) -> void { Screen.fill_rectangle(int(x), int(y), int(w), int(h), c) }
+function sc_text(x: float, y: float, s: string, size: float, c: int) -> void { Screen.draw_text(int(x), int(y), s, c, Math.max(1, int(size / 8.0))) }
+function sc_measure(s: string, size: float) -> float { return float(Screen.measure_text(s) * Math.max(1, int(size / 8.0))) }
+function sc_px() -> float { return float(Input.mouse_x()) }
+function sc_py() -> float { return float(Input.mouse_y()) }
+function sc_down() -> bool { return Input.mouse_down(0) }
+export function ui_screen_backend() -> UiBackend {
+  let b = new UiBackend
+  b.rect = fn sc_rect
+  b.text = fn sc_text
+  b.measure = fn sc_measure
+  b.pointer_x = fn sc_px
+  b.pointer_y = fn sc_py
+  b.pointer_down = fn sc_down
+  return b
+}