Add height zones, jetpack boost particles, and TigerStyle guidelines

Level generator: add vertical variety with tall levels (46 tiles).
Segment generators accept ground_row parameter, SEG_CLIMB connects
height zones, transitions inherit predecessor ground row to prevent
walkability gaps. Climb segments respect traversal direction.

Jetpack boost: add blue flame particles during dash (burst + trail)
and continuous idle glow from player back while boost timer is active.

Camera: add 30px vertical look-ahead when player velocity exceeds
50 px/s.

Fix flame vent pedestal in gen_pit to use ground-relative position
instead of map bottom (broken in tall HIGH-zone levels).

Add TigerStyle coding guidelines to AGENTS.md adapted for C11.
Add tall_test.lvl (40x46) for height zone validation.

src/game/player.c

@@ -288,6 +288,11 @@ void player_update(Entity *self, float dt, const Tilemap *map) {
         );
         particle_emit_jetpack_trail(exhaust_pos, pd->dash_dir);
 
+        /* Blue flame trail when boost is active */
+        if (pd->jetpack_boost_timer > 0) {
+            particle_emit_jetpack_boost_trail(exhaust_pos, pd->dash_dir);
+        }
+
         /* Skip normal movement during dash */
         physics_update(body, dt, map);
         animation_update(&self->anim, dt);
@@ -330,10 +335,28 @@ void player_update(Entity *self, float dt, const Tilemap *map) {
         );
         particle_emit_jetpack_burst(exhaust_pos, pd->dash_dir);
 
+        /* Blue flame accents when boost powerup is active */
+        if (pd->jetpack_boost_timer > 0) {
+            particle_emit_jetpack_boost_burst(exhaust_pos, pd->dash_dir);
+        }
+
         audio_play_sound(s_sfx_dash, 96);
         return;
     }
 
+    /* ── Jetpack boost idle glow ─────────────── */
+    /* Ambient blue flame from the player's back while boost is active
+     * and not dashing. Emits from the rear center of the sprite. */
+    if (pd->jetpack_boost_timer > 0) {
+        bool facing_left = (self->flags & ENTITY_FACING_LEFT) != 0;
+        Vec2 back_pos = vec2(
+            facing_left ? body->pos.x + body->size.x - 1.0f
+                        : body->pos.x + 1.0f,
+            body->pos.y + body->size.y * 0.45f
+        );
+        particle_emit_jetpack_boost_idle(back_pos, facing_left);
+    }
+
     /* ── Horizontal movement ─────────────────── */
     float target_vx = 0.0f;
     if (hold_left)  target_vx -= PLAYER_SPEED;