refactor(runtime,packages,examples): named constants, package enums, idiom sweep

- runtime: HEADLESS_FRAME_PATH, STICK_DEADZONE / STICK_LEFT_X/Y, key and
  byte codes as char literals throughout (`k == 'w'`, `fill(rt_map, ' ', …)`)
- ludic.gameplay/stats: drop the duplicate `stat_field` (it answered "atk"
  for every build stat); Stats.base uses stats_field_name
- ludic.shooter: compare aim modes and fire patterns with AimMode.* and
  WeaponPattern.* instead of raw ints; STICK_RIGHT_X/Y
- ludic.npcai: DecisionMade / brain_set_state use AiState.*
- examples/games/menu.ludic uses Font.load / Ui.* with FONT_PATH and
  BACKDROP named; strings.ludic header says what it prints
- whole tree: `x = x + 1` → `x += 1` (single-term right-hand sides only),
  `0 - x` → `-x`, ASCII codes → char literals; every .ludic and every
  ```ludic fence reformatted with the fixed formatter (whitespace only)

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

runtime/native/systems.ludic

@@ -60,7 +60,7 @@ function anim_clip(name: pointer, frames: int, fps: int, mode: int) -> void {
       anim_clip_frames[i] = frames;  anim_clip_fps[i] = fps;  anim_clip_mode[i] = mode
       return
     }
-    i = i + 1
+    i += 1
   }
   if anim_nclips >= ANIM_MAX_CLIPS { return }        # silently ignore past capacity
   let s = anim_nclips
@@ -68,7 +68,7 @@ function anim_clip(name: pointer, frames: int, fps: int, mode: int) -> void {
   anim_clip_frames[s] = frames
   anim_clip_fps[s]    = fps
   anim_clip_mode[s]   = mode
-  anim_nclips = anim_nclips + 1
+  anim_nclips += 1
 }
 
 function anim_clip_find(name: pointer) -> int {
@@ -76,9 +76,9 @@ function anim_clip_find(name: pointer) -> int {
   var i = 0
   while i < anim_nclips {
     if anim_clip_names[i] == name { return i }
-    i = i + 1
+    i += 1
   }
-  return 0 - 1
+  return -1
 }
 
 # ---- Anim.play / Anim.on_frame / Anim.fired (#48) --------------------------