Add bouncer launch pad to design document

DESIGN.md

@@ -88,6 +88,22 @@ Already implemented: `GRAVITY`, `WIND`, `BG_COLOR`, `MUSIC`, `PARALLAX_FAR`, `PA
 
 ---
 
+## Hazards & Mechanics
+
+### Implemented
+- **Flame Vent** — Floor-mounted grate, toggles on/off on a timer
+- **Force Field** — Vertical energy barrier, toggled by switch/timer
+- **Moving Platform** — Horizontal/vertical patrol between two points
+- **Laser Turret** — See Enemies above (also functions as a hazard)
+
+### Planned
+- **Bouncer** — Launch pad that shoots the player into the air on contact.
+  Two variants: straight (vertical impulse only) and angled (rotatable,
+  placed at arbitrary angles to launch the player diagonally or sideways).
+  Could also affect enemies and projectiles for puzzle potential.
+
+---
+
 ## Weapons / Projectiles
 
 Data-driven system: each weapon type is a `ProjectileDef` struct describing speed,