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>

examples/rejected/default_missing.ludic

@@ -0,0 +1,5 @@
+# L11: a call leaves out only what has a default
+program DefaultMissing {
+  function f(a: int, b: int = 2) -> int { return a + b }
+  entry { print(f(b: 3)) }
+}

examples/rejected/default_unknown.ludic

@@ -0,0 +1,5 @@
+# L11: a named argument names a parameter the function has
+program DefaultUnknown {
+  function f(a: int, b: int = 2) -> int { return a + b }
+  entry { print(f(1, c: 3)) }
+}

examples/rejected/positional_after_named.ludic

@@ -0,0 +1,5 @@
+# L11: positional arguments come first, then named ones
+program PositionalAfterNamed {
+  function f(a: int, b: int = 2) -> int { return a + b }
+  entry { print(f(a: 1, 2)) }
+}

examples/rejected/view_untyped.ludic

@@ -0,0 +1,9 @@
+# a view field that is not a bare global has to say its type
+import "ludic.ui"
+program ViewUntyped {
+  var a: int = 1
+  view Sums {
+    total = a + 2
+  }
+  entry { print(view_sums().name) }
+}

examples/rejected/view_without_ui.ludic

@@ -0,0 +1,8 @@
+# a view is for templates, which ludic.ui shows
+program ViewWithoutUi {
+  var a: int = 1
+  view Sums {
+    a = a
+  }
+  entry { print(a) }
+}