Language: named colors, namespaced Screen/Input/Random/Map API, named args

An opinionated, game-facing surface for the language:
- Color.<Name>: 221 named colors resolved to 0xRRGGBB at compile time
  (selfhost/emit_color.ludic).
- Namespaced builtins Screen.* / Input.key / Random.* / Map.*, so calls read as
  subject.action; present() -> Screen.show(), clear -> Screen.clear,
  fill_rect -> Screen.fill_rectangle, etc.
- Named arguments, e.g. Screen.fill_rectangle(x:, y:, width:, height:, color:),
  reordered to the callee's parameters at emit time.
- machine/become can run over a named program-scope var, not just a register.
- Migrate examples off numeric registers to named vars; snake/menu/chronorift
  render byte-identical to the goldens and the bootstrap fixpoint is preserved.
- Regenerate the checked-in seed (selfhost/ludicc.seed.ll).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

runtime/native/core.ludic

@@ -59,6 +59,9 @@ fn rt_shutdown() -> void {
 }
 
 # ---- framebuffer ----------------------------------------------------------
+fn rt_screen_w() -> int { return rt_fbw }
+fn rt_screen_h() -> int { return rt_fbh }
+
 fn rt_clear(c: int) -> void {
   let n = rt_fbw * rt_fbh
   for i in 0 .. n {