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>

selfhost/backend/emit_new.ludic

@@ -70,6 +70,11 @@ function emit_sized_slice(el: pointer, n: Val) -> Val {
   emit(`  store i32 {n.code}, ptr {slice_field(h.code, 2)}\n`)
   return h
 }
+# the file a runtime error names: the one the expression is written in, not the program's own
+function src_name_of(e: Node) -> pointer {
+  if e == null or e.file == null { return g_src_name }
+  return e.file
+}
 function emit_view(e: Node) -> Val {
   let xs = emit_expr(e.kids[0])
   if not is_slice_ty(xs.ty) { perr(`view takes a slice, and this is {xs.ty}`) }
@@ -88,7 +93,7 @@ function emit_view(e: Node) -> Val {
   emit(`  br i1 {ok}, label %{lok}, label %{lbad}\n`)
   emit(`{lbad}:\n`)
   g_uses_bounds = true
-  let bmsg = emit_str_const(`{g_src_name}:{itoa(e.line)}: view past the end: from `)
+  let bmsg = emit_str_const(`{src_name_of(e)}:{itoa(e.line)}: view past the end: from `)
   let bse = emit_bind(stdstream_rhs(2))
   emit(`  call i32 (ptr, ptr, ...) @fprintf(ptr {bse}, ptr @.fmt_bounds, ptr {bmsg}, i32 {endv}, i32 {ln})\n`)
   emit("  call void @exit(i32 1)\n  unreachable\n")