v0.3: Large world 4096x3072 + momentum waves + zoom-scaled lighting

- World expanded to 4096x3072 (16x area, tiled 4x4 copies)
- Active region margin increased to 200px
- Liquid momentum system (FLAG_MOMENTUM) for wave sloshing
- Zoom-at-cursor support (camera.zoomAt)
- Smoother zoom steps (0.2 per scroll tick)
- Light propagation scaled by zoom (passes & falloff)
- AGENTS.md updated with all current features

engine/src/grid.rs

@@ -9,6 +9,7 @@ pub struct Grid {
 
 pub const FLAG_UPDATED: u8 = 0b0000_0001;
 pub const FLAG_PLAYER: u8 = 0b0000_0010;
+pub const FLAG_MOMENTUM: u8 = 0b0000_0100;
 
 impl Grid {
     pub fn new(width: u32, height: u32) -> Self {