feat: prevent reversing directly into snake neck and play denied sound effect

AGENTS.md

@@ -37,7 +37,7 @@ snakePlus/
 ├── source/                     # C++ Implementation files
 │   ├── main.cpp                # Hardware setup, VBlank IRQ, timer ticks, game bootstrap
 │   ├── game_audio.cpp          # MaxMod soundbank loading, BGM loop, SFX dispatch
-│   ├── game_input.cpp          # Key mapping, inverted control logic, menu navigation
+│   ├── game_input.cpp          # Key mapping, inverted control logic, reverse-denial, menu navigation
 │   ├── game_logic.cpp          # Position update, edge wrapping, collision resolution, RNG
 │   └── game_render.cpp         # OAM sprite rendering, console text HUD & menu screens
 │
@@ -51,6 +51,7 @@ snakePlus/
 │   ├── 273792__pacomav__plop.wav                                  # Movement plop SFX
 │   ├── 332629__treasuresounds__item-pickup.wav                    # Apple pickup SFX
 │   ├── Boom.wav                                                   # Rotten / Cursed apple boom SFX
+│   ├── denied.wav                                                  # Reverse-direction denied thud SFX (synthesized)
 │   ├── 167126__crisstanza__unpause.wav                            # Unpause SFX
 │   ├── 167127__crisstanza__pause.wav                              # Pause SFX
 │   └── 73750__timbre__remix-of-benboncan-sad-trombone-more-wah-bright-de-clicked.wav # Fail SFX
@@ -95,6 +96,7 @@ The game operates in one of two scenes ([`Scene`](include/game_state.h#L9-L12)):
 - **Normal Apple (Red):** Spawns if count < 1. Awards +10 score, increases length by 1, plays pickup sound. Flashes/blinks every 4 ticks.
 - **Rotten Apple (Brown/Green):** Spawns in Hard Mode (periodic chance up to 20 on screen). Decreases length by 1, plays explosion SFX. If length drops below 2, triggers Game Over.
 - **Cursed Apple (Purple):** Spawns occasionally. Inverts player steering controls (Up ↔ Down, Left ↔ Right) for `CURSE_DURATION_TICKS` (40 ticks). The sprite is procedurally constructed at startup in [`main.cpp:91-112`](source/main.cpp#L91-L112) by copying the rotten apple tile pixels and mapping them to dedicated palette entries (`CURSE_PAL_SHADOW`, `CURSE_PAL_OUTLINE`, `CURSE_PAL_FILL`, `CURSE_PAL_GLOW`).
+- **Reverse-Direction Denial:** Pressing the D-Pad in the exact opposite direction (i.e. into the snake's own neck) is silently denied instead of causing an immediate Game Over. The game computes the would-be next head position (including edge-wrapping) and compares it to the neck segment; if they match, the direction change is rejected and a subtle "denied" thud SFX plays.
 
 ### 3.4 Game Modes
 - **Normal Mode:** Starts with length 4, timer frequency `TIMER_FREQ_1024(5)`, background color `RGB15(5,5,5)`.
@@ -108,7 +110,7 @@ The game operates in one of two scenes ([`Scene`](include/game_state.h#L9-L12)):
 |---|---|---|
 | **Menu** | D-Pad Up / Down | Change selected menu item |
 | **Menu** | A / START | Select / Toggle option / Start game |
-| **Playing** | D-Pad | Steer snake (inverted if cursed) |
+| **Playing** | D-Pad | Steer snake (inverted if cursed; reversing into neck is denied with subtle SFX) |
 | **Playing** | START | Pause / Unpause |
 | **Playing** | R | Swap LCD displays (`lcdSwap()`) |
 | **Playing** | SELECT | Toggle debug internals HUD overlay |