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>

changes/defaults-views-templates.md

@@ -0,0 +1,37 @@
+bump: minor
+type: feature
+**Default parameters, views and templates.** A parameter can have a default (`pad: float = 8.0`).
+A call leaves out what it does not change, and may pass its first arguments by position and the
+rest by name.
+
+A `view` declaration is the one bridge between a program and its UI. It names the fields a
+template may read, the functions it may ask and the `on` events it may send, and it writes
+`view_<name>() -> UiView`.
+
+`ludic.ui` is now a template runtime. Screens and components are XML files loaded at run time,
+with:
+- `{expression}` bindings;
+- `<if>`, `<else>` and `<each>`;
+- props, `<slot/>` and per-instance `<state>`;
+- `on-press` actions that send events, `set` state or `emit` to the component's user;
+- component libraries (`export="true"`, `<import src as>`);
+- HTML's elements (`div`, `p`, `h1`-`h6`, `ul`/`li`, `img`, `hr`, ...), with a default stylesheet;
+- HTML's attributes: `id`, `class`, `style`, `hidden`, `disabled` and `onclick`, with any other
+  attribute kept for selectors;
+- the CSS box model (padding and margin in 1-4 values, borders, `px` and `%`) and flex layout
+  (`flex-grow`, `justify-content`, `align-items`/`align-self`, `flex-wrap`, min and max sizes)
+  under CSS's property names;
+- stylesheets, in a `<style>` or an `.lss` file (a Ludic StyleSheet) that others import and that
+  can `@import` more;
+- CSS's selectors: `#id`, compound classes, `[attr=value]`, descendant and `>` combinators,
+  `:hover`, `:disabled`, `:first-child`, `:last-child`, `:nth-child`, `:not` and more, weighed by
+  specificity.
+
+Screens are drawn through a registered renderer. `Value` gains a float kind.
+
+Also:
+- A program's function named like one of the runtime's is refused; it had been silently taking the
+  runtime's own calls.
+- An index is evaluated before the slice's elements are read. A `xs[f()]` whose `f` grew `xs`
+  read stale memory.
+- A runtime error names the file its expression is in, not the program's.