docs: document hardware constraints, ISR decoupling, and GDB troubleshooting

Document the 256-byte ARM9 IRQ stack limitation, console non-reentrancy,
input bus contention avoidance with swiWaitForVBlank, boot sequencing,
HUD dirty-flag caching, and GDB Data Abort diagnosis procedures.
Update README with progressive speed scaling and persistent Game Over screen.

Co-authored-by: Antigravity <antigravity@google.com>

README.md

@@ -19,9 +19,15 @@
   - Background tracker music ([`0rigin.xm`](sounds/0rigin.xm)) powered by MaxMod 9.
   - Sound effects for movement, pickups, rotten/cursed explosions, pause, and game over.
   - On-the-fly music toggle from the menu or pause screen.
-- **Game Modes:**
-  - **Normal Mode:** Relaxed tick rate (`TIMER_FREQ_1024(5)`), starting length 4.
-  - **Hard Mode:** High-speed tick rate (`TIMER_FREQ_1024(9)`), starting length 2, and periodic rotten apple hazards.
+- **Game Modes & Progressive Intensity:**
+  - **Normal Mode:** Starts at 5.0 Hz, scaling smoothly up to 13.0 Hz as apples are consumed.
+  - **Hard Mode:** Starts at 5.0 Hz and scales up to a blazing 17.0 Hz, spawning rotten apple hazards and dynamic temporary energy barriers.
+  - **Temporary Fatal Barriers:** Energy walls appear dynamically in Hard Mode after 3 apples. Each barrier flashes a warning reticle for 2.0 s before solidifying into a lethal hazard-striped obstacle for 4.0 s.
+  - **Dynamic Backgrounds:** Dual-screen cyber grids accelerate drift and intensify color palettes (plasma cyan in Normal, hyper-crimson in Hard, radioactive violet when cursed) in tandem with game speed.
+- **Persistent Game Over Screen:**
+  - Collision state is preserved: snake, apples, and barriers remain visible on the playfield so players can review their final position.
+  - Background smoothly decelerates and dims into a somber tone.
+  - Dedicated top-screen summary presents final score, apples eaten, length, difficulty, and exact cause of defeat (Bite Collision, Energy Barrier Impact, or Rotten Apple Depletion).
 - **Main Menu & Pause Screen:**
   - Integrated start menu to toggle Hard Mode and Music.
   - In-game pause menu with instant restarts and mode toggles.