feat: add temporary hazardous barriers in hard mode

AGENTS.md

@@ -102,8 +102,9 @@ The game operates in one of two scenes ([`Scene`](include/game_state.h#L9-L12)):
 
 ### 3.4 Game Modes & Progressive Speed Mechanics
 - **Normal Mode:** Starts with length 4, base timer frequency 5.0 Hz (`SPEED_BASE_NORMAL_MHZ = 5000`), scaling up to 13.0 Hz (`SPEED_MAX_NORMAL_MHZ = 13000`).
-- **Hard Mode:** Starts with length 2, base timer frequency 9.0 Hz (`SPEED_BASE_HARD_MHZ = 9000`), scaling up to 17.0 Hz (`SPEED_MAX_HARD_MHZ = 17000`), and spawns rotten apples.
-- **Progressive Speed Scaling:** Each consumed regular apple increases `state.applesEaten`. Game speed scales linearly with consumed apples up to `SPEED_APPLES_CAP = 40`, granting smooth +200 mHz acceleration per apple. Timer 0 reload registers are recomputed on the fly (`calculate_timer_reload()`).
+- **Hard Mode:** Starts with length 2, base timer frequency 5.0 Hz (`SPEED_BASE_HARD_MHZ = 5000`, matching Normal Mode's starting speed), scaling up to 17.0 Hz (`SPEED_MAX_HARD_MHZ = 17000`), spawns rotten apples, and spawns temporary fatal barriers.
+- **Temporary Fatal Barriers (Hard Mode):** Straight lines of blocks appear dynamically across the playfield without overlapping the snake or apples. Barriers only begin spawning after a few apples have been eaten (`BARRIER_START_APPLES = 3`). Their maximum possible length begins at 4 blocks (`BARRIER_BASE_MAX_LENGTH`) and gradually scales up to 10 blocks (`BARRIER_MAX_LENGTH`) later on in the game. At later stages of the game, multiple barriers can spawn simultaneously in a single spawn event (up to 2 at once starting at 16 apples, and up to 3 at once starting at 30 apples), synchronizing their warning and activation phases. Before becoming lethal, each barrier announces itself with a flashing warning reticle frame (`BARRIER_WARN_FRAMES = 120` frames / 2.0 s) whose pulse frequency accelerates and warning color intensifies from dim orange to incandescent yellow-white as solidification approaches. Once active, barriers turn into solid hazard-striped energy walls (`BARRIER_ACTIVE_FRAMES = 240` frames / 4.0 s) fatal to the snake's head on contact. As apples are eaten, spawn intervals accelerate from 9.0 s down to 2.0 s (`BARRIER_INTERVAL_MAX_FRAMES` to `BARRIER_INTERVAL_MIN_FRAMES`) and max concurrent barriers scale up from 1 to 4 (`BARRIER_MAX_COUNT`).
+- **Progressive Speed Scaling:** Each consumed regular apple increases `state.applesEaten`. Game speed scales linearly with consumed apples up to `SPEED_APPLES_CAP = 40`, granting smooth +200 mHz acceleration per apple (or +300 mHz in Hard Mode). Timer 0 reload registers are recomputed on the fly (`calculate_timer_reload()`).
 - **Progressive Background Intensity Scaling:** Visual effects intensify at the exact same rate (`calculate_intensity_31()`):
   - Hardware drift scroll speeds accelerate on both top and bottom screens.
   - Celestial node oscillation & twinkling breathing frequencies increase via fixed-point phase accumulation (`anim_phase_fp`).