feat(ui): ludic.ui grows toward HTML, CSS and React (Phase 0.U1-U4)

- Natives: ui_native(tag, measure, draw) makes an element the program draws itself; ui_fire
  runs its on-<event> with event.value. input, select and textarea have simple defaults.
- React:
  - keyed <each>, <let>, <provide> context through components, <fragment>;
  - named slots, default props on <component>;
  - on-mount / on-unmount;
  - inline text inside text elements.
- CSS:
  - custom properties and var();
  - position relative/absolute/fixed with insets and z-index;
  - em/rem/vw/vh and @media;
  - wrapping text and ellipsis, overflow;
  - + and ~ combinators, :nth-child(an+b), :checked, :active.
- Developing: errors with file:line, ui_errors(), ui_reload() keeping state, and an
  inspector-style ui_dump.
- view fields infer the type of a literal or a named function's result.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.ui/env.ludic

@@ -12,6 +12,7 @@ export property UiEnv {
   on_names: []string = null      # a component's on-* attributes, which its `emit` runs
   on_acts: []UiAct = null
   on_envs: []UiEnv = null
+  ctx: UiEnv = null              # what a <provide> above gave, seen through components too
 }
 var ui_view: UiView = null
 var ui_errs: []string = new []string
@@ -21,7 +22,10 @@ function env_new(parent: UiEnv) -> UiEnv {
   e.names = new []string
   e.vals = new []Val
   e.parent = parent
-  if parent != null { e.model = parent.model }
+  if parent != null {
+    e.model = parent.model
+    e.ctx = parent.ctx
+  }
   return e
 }
 function env_bind(e: UiEnv, name: string, v: Val) -> void {
@@ -39,6 +43,13 @@ function env_get(e: UiEnv, name: string) -> Val {
     if at.state != null and Value.has(at.state, name) != 0 { return Value.get(at.state, name) }
     at = at.parent
   }
+  var cx = e.ctx
+  while cx != null {
+    for i in 0 .. len(cx.names) {
+      if cx.names[i] == name { return cx.vals[i] }
+    }
+    cx = cx.parent
+  }
   if e.model != null and Value.has(e.model, name) != 0 { return Value.get(e.model, name) }
   ui_err(`no {name} here: not a loop's, a prop, a state or in the view`)
   return Value.null()
@@ -55,11 +66,24 @@ function env_set(e: UiEnv, name: string, v: Val) -> void {
   }
   ui_err(`set {name}: no state here holds it - declare it with <state {name}="..."/>`)
 }
-# an error is said once, however many frames make it
-function ui_err(msg: string) -> void {
+# an error is said once, however many frames make it, and names the template line being built
+var ui_at: string = ""
+# said by the runtime, or by a host about something a template asked for
+export function ui_err(msg0: string) -> void {
+  var msg = msg0
+  if ui_at != "" and not ui_has(msg, ".xml:") { msg = `{ui_at}: {msg}` }
   for i in 0 .. len(ui_errs) {
     if ui_errs[i] == msg { return }
   }
   push(ui_errs, msg)
   print(`ui: {msg}`)
 }
+function ui_has(s: string, part: string) -> bool {
+  let n = len(part)
+  for i in 0 .. len(s) - n + 1 {
+    if s[i..i + n] == part { return true }
+  }
+  return false
+}
+# every error said so far - for a host to show them where a developer will see them
+export function ui_errors() -> []string { return ui_errs }