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>

tools/ludic-cli/selfhost.ludic

@@ -23,6 +23,10 @@ function selfhost_frags() -> []pointer {
   push(f, "selfhost/frontend/registry.ludic")
   push(f, "selfhost/frontend/registry_finish.ludic")
   push(f, "selfhost/frontend/resource.ludic")
+  push(f, "selfhost/frontend/defaults.ludic")
+  push(f, "selfhost/frontend/view.ludic")
+  push(f, "selfhost/frontend/view_conv.ludic")
+  push(f, "selfhost/frontend/view_finish.ludic")
   push(f, "selfhost/backend/emit_core.ludic")
   push(f, "selfhost/backend/emit_head.ludic")
   push(f, "selfhost/backend/emit_addr.ludic")

tools/ludic-cli/test.ludic

@@ -724,7 +724,7 @@ function cmd_dev_test() -> int {
   feat_case("modules/visible", "", "42 42", "visible.ludic (L3: a module's exported function and event used from outside it)")
   reject_case("rejected/private_module", "add is private to module bank", "a private function of another module is refused")
   feat_case("lang/checked", "", "7.5 3 12 hi 2 ok", "checked.ludic (L4: literals take their slot's kind, string(p), []string as []pointer, null, named args)")
-  reject_case("rejected/wrong_arity", "area takes 2 argument(s) and this call gives 1", "a call with the wrong number of arguments is refused")
+  reject_case("rejected/wrong_arity", "this call to area leaves out h, which has no default", "a call with the wrong number of arguments is refused")
   reject_case("rejected/float_into_int", "metres wants an int and this is a float", "a float computed into an int is refused")
   reject_case("rejected/record_mixup", "d wants a Door and this is a Key", "one record type is not another")
   reject_case("rejected/record_as_text", "argument 1 of sign wants a string and this is a Trail", "a record is not text")
@@ -736,7 +736,7 @@ function cmd_dev_test() -> int {
   reject_case("rejected/generic_arity", "Pool takes 1 type argument(s) and Pool<int, string> gives 2", "a generic takes as many type arguments as it has parameters")
   reject_case("rejected/private_generic", "grow is private to module kit", "an instance of a private generic is private to its module")
   feat_case("lang/aliases", "", "10 5 3", "aliases.ludic (L6: a namespace method declared as an alias of a function, labelled or by its parameters)")
-  reject_case("rejected/alias_arity", "Trail.length takes 2 argument(s) and this call gives 1", "an alias's arguments are checked against its target")
+  reject_case("rejected/alias_arity", "this call to Trail.length leaves out to, which has no default", "an alias's arguments are checked against its target")
   reject_case("rejected/alias_twice", "Trail.length is declared twice", "a namespace method is declared once")
   feat_case("lang/registry", "", "3 Crate true 2 fire_ring -1", "registry.ludic (L8: a registry filled by def declarations - index constants, a count, the key, a lookup)")
   reject_case("rejected/registry_unknown", "there is no registry Tables", "a def names a registry that exists")
@@ -745,6 +745,15 @@ function cmd_dev_test() -> int {
   feat_case("lang/resources", "", "2 Axe 2 Split 10 0 1", "resources.ludic (L9: a registry read from a resource file at compile time, nested records typed by the schema)")
   reject_case("rejected/resource_field", "bad_tools.lres:3: error: field weight of Tool wants a float", "a resource file's entry is checked at its own line")
   reject_case("rejected/resource_missing", "cannot read the resource file data/no_such.lres", "a registry's resource file must exist")
+  feat_case("lang/defaults", "", "a 300.0 8.0 / b 120.0 8.0 / c! 300.0 8.0 / d 300.0 2.0 / x / y", "defaults.ludic (L11: default parameters, positional then named arguments, in a generic too)")
+  reject_case("rejected/default_missing", "this call to f leaves out a, which has no default", "a call leaves out only what has a default")
+  reject_case("rejected/default_unknown", "f has no parameter c", "a named argument names a parameter the function has")
+  reject_case("rejected/positional_after_named", "a positional argument after a named one", "positional arguments come first")
+  controller_case("library/ui_templates", "", "ui: ui/peek.xml: <kit:Price>: that component is not exported - export=\"true\" on it lets other files use it purse 70, bought 1; the lantern is enabled: 0", "ui_templates.ludic (L11: a template reads a view, a press sends its event, component state and emit)")
+  controller_case("library/ui_flex", "", "grow 97:282; between at 197; wrap lines at 51, 67; red 16711680; stretched 40; own size 12", "ui_flex.ludic (L11: flex layout - grow, justify, wrap, stretch - and stylesheets with imports and specificity)")
+  controller_case("library/ui_html", "", "id beats class: 40; card 200 wide at 3,53 pad 4 8 12 16 border 2 inside 65280, outside -1; warn bg 16776960; items 3 of 30: 30; colours 16711680 255 -1; hr 1; children 7 clicks 1; the button now disabled 1 at opacity 0.5; text-align 1", "ui_html.ludic (L11: HTML elements, ids, classes, attributes, style=, CSS selectors and properties, the box model)")
+  reject_case("rejected/view_untyped", "view Sums: say what type total is", "a view field that is not a bare global says its type")
+  reject_case("rejected/view_without_ui", "a view is shown by ludic.ui", "a view needs ludic.ui to show it")
 
   # EV2 the world table: the mod reflection ABI, callable from Ludic by name.
   net_case("ecs/world_get", "50 1 7")