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_addr.ludic

@@ -50,13 +50,16 @@ function emit_index_addr(e: Node) -> pointer {
     return r
   }
   let el = slice_elem(base.ty)
+  # the index is evaluated BEFORE the elements' address is read: evaluating it may grow this
+  # very slice (xs[slot_of(k)] where slot_of pushes), and a push can move the elements
+  let ix = emit_expr(e.b)
   # load the data pointer from the slice header (field 0)
   let dp = nreg()
   emit("  ");  emit(dp);  emit(" = getelementptr inbounds %LSlice, ptr ")
   emit(base.code);  emit(", i32 0, i32 0\n")
   let data = nreg()
   emit("  ");  emit(data);  emit(" = load ptr, ptr ");  emit(dp);  emit("\n")
-  let ix = emit_expr(e.b)                          # (evaluate index BEFORE setting
+  # (the index was evaluated first, above, and before g_addr_ty is set
   # THE INDEX IS CHECKED AGAINST THE LENGTH. A slice has carried { ptr, len, cap } since it
   # existed, and nothing ever read the len: every index was a bare getelementptr, so running
   # off the end of one wrote into whatever the allocator had put next and the program died
@@ -73,7 +76,7 @@ function emit_index_addr(e: Node) -> pointer {
     let lbad = lbl("ixbad")
     emit(`  br i1 {okc}, label %{lok}, label %{lbad}\n`)
     emit(`{lbad}:\n`)
-    let bmsg = emit_str_const(`{g_src_name}:{itoa(e.line)}: index out of range: `)
+    let bmsg = emit_str_const(`{src_name_of(e)}:{itoa(e.line)}: index out of range: `)
     let bse = emit_bind(stdstream_rhs(2))
     emit(`  call i32 (ptr, ptr, ...) @fprintf(ptr {bse}, ptr @.fmt_bounds, ptr {bmsg}, i32 {ix.code}, i32 {lnv})\n`)
     emit("  call void @exit(i32 1)\n  unreachable\n")