Phase 6e: unify builtin naming (loaders domain-first, set_reg)

Fixed the naming inconsistencies the cohesion audit flagged, converging on the
domain-first style the bulk of the surface already uses (ui_*, text_*, rng_*,
font_load, image_load):

  - load_png    -> png_load       (asset loaders were split: font_load/image_load
  - load_sprites -> sprites_load   were domain-first, load_* were verb-first)
  - setreg      -> set_reg        (missing underscore vs ui_set_int/ui_set_text)

Game builtins resolve to their `rt_` runtime function, so the renames are in
runtime/native (rt_png_load, rt_sprites_load, rt_set_reg) plus the ~100 example
call sites; `become` lowering in emit_machine now emits @fn_rt_set_reg. Purely a
surface rename — every renamed call maps to the same runtime symbol, so behavior
and golden renders are byte-identical.

Vocabulary + docs updated (ludic_syntax.h, grammar, LudicTokens.kt, LANGUAGE.md,
README, SYNTAX-REDESIGN). Reseeded; C-free fixpoint holds; goldens identical;
17/17; vocab clean.

Left as-is: os_argc/os_arg (compiler-internal intrinsics, already namespaced and
consistent with each other; renaming would need a bootstrap dance for little
gain). reg/set_reg keep the getter-bare/setter-set_ shape ui_focused/ui_set_int
already use.

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

runtime/native/core.ludic

@@ -178,7 +178,7 @@ fn rt_reg(i: int) -> int {
   return peek32(rt_regs, i)
 }
 
-fn rt_setreg(i: int, v: int) -> void {
+fn rt_set_reg(i: int, v: int) -> void {
   if i < 0 { return }
   if i >= 64 { return }
   poke32(rt_regs, i, v)