docs: correct stale references across LANGUAGE, README, COMPILING, CONTRIBUTING

- LANGUAGE.md: real diagnostic format, list literals, Font.load / Ui.*
  (no `reg`/`set_reg`, no `/Handler/Library` typo), `extern function`,
  existing example links, Input.key(); builtins table trimmed to what exists
- COMPILING.md: pipeline names selfhost/ (not compiler/*.c), `program`
  instead of game/module, all thirteen win_* entry points by group
- README.md: the window seam is not "five" functions
- CONTRIBUTING.md: docs are checked with `x docs-gen` / `x docs-check`
- docs/language: kw-ui and fn-ui_build use the namespaced API;
  @ClearColor documents constant expressions; examples/README lists
  operators.ludic

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/site/snippets/hero.ludic

@@ -13,7 +13,7 @@ program Hello {
   # runs once per match, each property bound by name.
   @Queries(these: [Position, Velocity])
   handler AdvancePositions phase FixedUpdate {
-    Position.column = Position.column + Velocity.delta_x
+    Position.column += Velocity.delta_x
     Position.row    = Position.row + Velocity.delta_y
   }
 

docs/site/snippets/scenes.ludic

@@ -11,7 +11,7 @@ program SceneDemo {
     on exit  { print(2) }
     layer Main {
       handler Tick phase Update {
-        counter = counter + 1
+        counter += 1
         print(100 + counter)
         if counter >= 2 { become Play }        # hand off to Play
       }
@@ -26,7 +26,7 @@ program SceneDemo {
     on exit { print(4) }
     layer World {
       handler Step phase Update {
-        counter = counter + 1
+        counter += 1
         print(200 + counter)
         if counter >= 2 { quit() }
       }