Implement run mechanic and level progression with top-left HUD indicator

Run (B + direction):
- player_logic.c: while holding B + direction with stamina >= 10, each tile
  takes 8 frames instead of 16 (move_progress += 2, LERP >>4 unchanged) and
  costs 10 stamina per tile. DAS_REPEAT_RUN=2 chains tiles fluidly.
- Falls back silently to normal walk when stamina < 10.

Level progression:
- New global 'level' (starts at 1). title->game resets to 1; reaching the
  hatch (victory/Going Deeper) + START increments before engine_init (new
  maze); defeat + START resets to 1.
- Top-left HUD 'L<n>' via 3 sprites (OAM 23-25) from new level.png asset
  (glyphs L, 0-9), generated by scripts/generate_level.py and converted
  with png2asset -keep_duplicate_tiles for predictable tile ordering.
- VRAM base aligned to an EVEN index (8x16 hardware ignores tile LSB) and
  placed after the stamina load claim to avoid the shared BG tile block.
- update_level_display() called every frame; hides during game over/title.

Docs: README, doc/gameplay.md and the technical report updated.

src/main.c

@@ -1,5 +1,6 @@
 #include <gb/gb.h>
 #include "engine.h"
+#include "globals.h"
 
 uint8_t app_state = 0; // 0 = title, 1 = game
 
@@ -20,6 +21,7 @@ void main(void) {
             title_update(keys, prev_keys);
             if ((keys & J_START) && !(prev_keys & J_START)) {
                 app_state = 1;
+                level = 1; // Nuova partita dal livello 1
                 engine_init();
             }
         } else {