ui: a renderer installs itself through the UiRenderers registry, not a global's initializer

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

packages/ludic.ui/backend.ludic

@@ -20,7 +20,17 @@ export property UiBackend {
   now: fn() -> float = null                # seconds, for animation (else frames at 60 a second)
   input: bool = true               # false: take no input from Input.* (a test drives it)
 }
-var ui_be: UiBackend = null
+# a renderer a program installs by importing it (render3d.ludic): the first one is the backend
+property UiRenderer {
+  key: string = ""
+  make: fn() -> UiBackend = null
+}
+registry UiRenderers of UiRenderer as UIR
+function ui_default_backend() -> UiBackend {
+  if len(UiRenderers) == 0 { return null }
+  return UiRenderers[0].make()
+}
+var ui_be: UiBackend = ui_default_backend()
 export function ui_backend(b: UiBackend) -> void { ui_be = b }
 function ui_px(v: float) -> float {
   if ui_be != null and ui_be.scale != null { return v * ui_be.scale() }

packages/ludic.ui/render3d.ludic

@@ -17,13 +17,10 @@ var r3u_atlas_tex: []int = new []int
 var r3u_atlas_cols: []int = new []int
 var r3u_atlas_rows: []int = new []int
 var r3u_atlas_names: [][]string = new [][]string
-var r3u_installed: bool = r3u_install()
 # importing this file is enough: it is the renderer from the program's first frame
-function r3u_install() -> bool {
-  ui_render3d()
-  return true
-}
-export function ui_render3d() -> void {
+def UiRenderers render3d { make: fn r3u_make }
+export function ui_render3d() -> void { ui_backend(r3u_make()) }
+function r3u_make() -> UiBackend {
   let b = new UiBackend
   b.rect = fn r3u_rect
   b.round = fn r3u_round
@@ -38,7 +35,7 @@ export function ui_render3d() -> void {
   b.unclip = fn r3u_unclip
   b.scale = fn r3u_scale
   b.now = fn r3u_now
-  ui_backend(b)
+  return b
 }
 # the player's interface size, 1.0 as designed
 export function ui_render3d_scale(s: float) -> void { r3u_user = s }