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/base64.ludic

@@ -15,12 +15,12 @@
 # the standard alphabet value of a base64 character, or -1 for anything else
 # (whitespace, '=', stray bytes — the decoder skips them / stops on '=').
 function b64_val(c: int) -> int {
-  if c >= 65 and c <= 90  { return c - 65 }          # 'A'..'Z' -> 0..25
-  if c >= 97 and c <= 122 { return c - 71 }          # 'a'..'z' -> 26..51
-  if c >= 48 and c <= 57  { return c + 4 }            # '0'..'9' -> 52..61
-  if c == 43 { return 62 }                            # '+'
-  if c == 47 { return 63 }                            # '/'
-  return 0 - 1
+  if c >= 'A' and c <= 'Z'  { return c - 65 }          # 'A'..'Z' -> 0..25
+  if c >= 'a' and c <= 'z' { return c - 71 }          # 'a'..'z' -> 26..51
+  if c >= '0' and c <= '9'  { return c + 4 }            # '0'..'9' -> 52..61
+  if c == '+' { return 62 }                            # '+'
+  if c == '/' { return 63 }                            # '/'
+  return -1
 }
 
 # decode NUL-terminated base64 `src` into the caller's `out` buffer; returns the
@@ -33,18 +33,18 @@ function b64_decode(src: pointer, out: pointer) -> int {
   var i = 0
   var c = src[i]
   while c != 0 {
-    if c == 61 { return w }                            # '=' -> done
+    if c == '=' { return w }                            # '=' -> done
     let v = b64_val(c)
     if v >= 0 {
       acc = (acc << 6) | v
-      nbits = nbits + 6
+      nbits += 6
       if nbits >= 8 {
-        nbits = nbits - 8
+        nbits -= 8
         out[w] = (acc >> nbits) & 255
-        w = w + 1
+        w += 1
       }
     }
-    i = i + 1
+    i += 1
     c = src[i]
   }
   return w
@@ -79,9 +79,9 @@ function base64_encode(src: pointer) -> pointer {
     let e2 = ((b1 & 15) << 2) | (b2 >> 6)
     let e3 = b2 & 63
     out = out + tab[e0..e0 + 1] + tab[e1..e1 + 1]
-    if have >= 2 { out = out + tab[e2..e2 + 1] } else { out = out + "=" }
-    if have >= 3 { out = out + tab[e3..e3 + 1] } else { out = out + "=" }
-    i = i + 3
+    if have >= 2 { out += tab[e2..e2 + 1] } else { out += "=" }
+    if have >= 3 { out += tab[e3..e3 + 1] } else { out += "=" }
+    i += 3
   }
   return out
 }