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>

examples/games/snake.ludic

@@ -79,7 +79,7 @@ program Snake {
 
   handler Move phase Update {
     if mode == MODE_PLAY {
-      tick = tick + 1
+      tick += 1
       if tick >= 6 {
         tick = 0
         # remember the tail cell, then drag every segment onto the one ahead of it
@@ -102,10 +102,10 @@ program Snake {
         ate = 0
         for (ph, sh) in query [Pos, Seg] {
           if sh.order == 0 {
-            if direction == DIR_UP    { ph.y = ph.y - 1 }
-            if direction == DIR_DOWN  { ph.y = ph.y + 1 }
-            if direction == DIR_LEFT  { ph.x = ph.x - 1 }
-            if direction == DIR_RIGHT { ph.x = ph.x + 1 }
+            if direction == DIR_UP    { ph.y -= 1 }
+            if direction == DIR_DOWN  { ph.y += 1 }
+            if direction == DIR_LEFT  { ph.x -= 1 }
+            if direction == DIR_RIGHT { ph.x += 1 }
             if ph.x < 0       { mode = MODE_OVER }
             if ph.x >= GRID_W { mode = MODE_OVER }
             if ph.y < 0       { mode = MODE_OVER }
@@ -124,9 +124,9 @@ program Snake {
           }
         }
         if ate == 1 {
-          score = score + 1
+          score += 1
           spawn Body { Seg { order: length };  Pos { x: tail_x, y: tail_y } }
-          length = length + 1
+          length += 1
           food_x = Random.range(low: 0, high: GRID_W - 1)
           food_y = Random.range(low: 0, high: GRID_H - 1)
         }