Grow maze size with level (7x7 -> 17x17, +2 per level)

The maze side length is now a runtime global 'map_size' (MAP_SIZE=7 at
level 1, +2 per level, capped at MAX_MAP_SIZE=17), while the maze array
is allocated with the MAX_MAP_SIZE bound. All modules (maze DFS, fog
rendering, player/enemy bounds, hatch and enemy-spawn placement) now use
map_size at runtime.

- globals: maze[MAX_MAP_SIZE][MAX_MAP_SIZE], map_size global, MAX_MAP_SIZE=17.
- maze.c: DFS on map_size; stack/valid arrays made static (WRAM) and sized
  for the max maze to avoid hardware-stack overflow; hatch min distance
  scales with map_size/2.
- engine.c: sets map_size from level before generate_maze; enemy spawn min
  distance scales with map_size/2.
- render.c: flush the full 32x32 background map (the old rows 2-17
  optimization broke for large mazes where the 5x5 fog window wraps outside
  that range); dynamic bounds; signed/unsigned casts.
- player_logic/enemy_logic: dynamic map_size bounds.

Verified via PyBoy: sizes 7,9,11,13,15,17,17 across levels 1-7, each with
a hatch; rendering and movement intact at 17x17 and at 7x7.

src/globals.c

@@ -4,7 +4,8 @@
 volatile uint8_t game_over = 0;
 volatile uint8_t game_over_timer = 0;
 
-uint8_t maze[MAP_SIZE][MAP_SIZE];
+uint8_t map_size = MAP_SIZE;
+uint8_t maze[MAX_MAP_SIZE][MAX_MAP_SIZE];
 uint8_t map_buffer[32 * 32];
 
 uint8_t player_lx = 1;