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>

README.md

@@ -167,7 +167,7 @@ Controls:
 - [x] Cross-compilation to ELF (x86-64, aarch64) and Windows COFF
 - [x] ECS runtime (properties, systems, phases, queries, entity pooling)
 - [x] Windowed 2D rendering (Cocoa driven from LLVM IR) + headless PPM verification
-- [x] CC0 Kenney PNG sprites (`load_png`, decoder written in Ludic) + scrolling camera
+- [x] CC0 Kenney PNG sprites (`png_load`, decoder written in Ludic) + scrolling camera
 - [x] Overworld: tilemap, movement, collision
 - [x] Random encounters + turn-based battle (HP/MP, seeded-RNG damage)
 - [x] Party + **local co-op** (P1 Knight, P2 Mage, per-player turns)