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/backend.ludic

@@ -0,0 +1,26 @@
+# backend.ludic - the renderer seam
+# the renderer. Only rect, text and measure are needed; the rest improve on a default:
+# button draws, focuses and answers a press itself; color names a colour (else #rrggbb);
+# image draws an <img src>; px scales a template's lengths; scroll_begin answers how far a region is scrolled; read
+# reads a template file (else Fs.read_text)
+export property UiBackend {
+  rect: fn(float, float, float, float, int, float) -> void = null
+  text: fn(float, float, string, float, int) -> void = null
+  measure: fn(string, float) -> float = null
+  pointer_x: fn() -> float = null
+  pointer_y: fn() -> float = null
+  pointer_down: fn() -> bool = null
+  button: fn(float, float, float, float, string, bool) -> bool = null
+  image: fn(string, float, float, float, float, int, float) -> void = null   # src x y w h tint alpha
+  color: fn(string) -> int = null
+  px: fn(float) -> float = null
+  scroll_begin: fn(string, float, float, float, float, float) -> float = null
+  scroll_end: fn() -> void = null
+  read: fn(string) -> string = null
+}
+var ui_be: UiBackend = null
+export function ui_backend(b: UiBackend) -> void { ui_be = b }
+function ui_px(v: float) -> float {
+  if ui_be != null and ui_be.px != null { return ui_be.px(v) }
+  return v
+}