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/render_buffer.rs

@@ -7,6 +7,7 @@ pub struct RenderBuffer {
     pub height: u32,
     light: Vec<u8>,
     block: Vec<u8>,
+    zoom: f32,
 }
 
 impl RenderBuffer {
@@ -19,6 +20,7 @@ impl RenderBuffer {
             height,
             light: vec![0; lsize],
             block: vec![0; lsize],
+            zoom: 1.0,
         }
     }
 
@@ -27,6 +29,8 @@ impl RenderBuffer {
         let rh = self.height as i32;
         let total = (rw * rh) as usize;
 
+        self.zoom = zoom;
+
         for i in 0..total {
             self.light[i] = 0;
             self.block[i] = 0;
@@ -49,7 +53,8 @@ impl RenderBuffer {
             }
         }
 
-        for _pass in 0..12 {
+        let max_passes = (16.0 / zoom).max(6.0).min(24.0) as u32;
+        for _pass in 0..max_passes {
             let dir = _pass & 1;
             let mut changed = false;
             if dir == 0 {
@@ -157,7 +162,8 @@ impl RenderBuffer {
                 let ni = (ny * rw + nx) as usize;
                 let nlight = self.light[ni] as u32;
                 if nlight <= 1 { continue; }
-                let falloff = (if nx == px || ny == py { 2 } else { 3 }) + blk;
+                let base_falloff = if nx == px || ny == py { 2.0 } else { 3.0 };
+                let falloff = (base_falloff / self.zoom) as u32 + blk;
                 if nlight > falloff {
                     let incoming = nlight - falloff;
                     if incoming > best {