docs: add Phase 4 documentation for magnet layouts

- Document magnet layout configuration
- Add metadata filtering examples
- Show use cases for attractors/repellers
- Document all magnet configuration options

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: Max Carlson

WebSites/spacecraft-viewer/README.md

@@ -30,6 +30,14 @@ Three.js-based spatial visualization viewer with D3-like declarative API.
 - ✅ ForceLayout with spring-based physics
 - ✅ Type-safe extensibility system
 
+## Features (Phase 4)
+
+- ✅ Rapier physics engine integration
+- ✅ Magnet system (attractors/repellers)
+- ✅ Metadata-driven force parameters
+- ✅ Tag-based filtering for magnets
+- ✅ Strength scaling by metadata fields
+
 ## Quick Start
 
 ```typescript
@@ -95,6 +103,40 @@ viewer.start()
 })
 ```
 
+**Magnet Layout** (Phase 4) - Physics-based layout with metadata filtering
+```typescript
+await viewer
+  .data(items)
+  .into('items', {
+    layout: 'magnet',
+    magnets: [
+      {
+        position: { x: 30, y: 0, z: 0 },
+        strength: 50,
+        radius: 100,
+        type: 'attractor',
+        filter: {
+          field: 'tags',
+          value: 'cyberpunk',
+          operator: 'contains'
+        }
+      }
+    ],
+    iterations: 100,
+    damping: 0.95
+  })
+```
+
+**Magnet Configuration Options:**
+- `position`: 3D position of the magnet
+- `strength`: Force magnitude
+- `radius`: Maximum influence distance
+- `type`: 'attractor' or 'repeller'
+- `falloff`: 'linear', 'inverse', or 'inverse-square' (default)
+- `filter`: Filter by metadata field (equals, contains, greater-than, less-than)
+- `strengthField`: Scale force by metadata field value
+- `strengthScale`: Multiplier for strength scaling
+
 ### Custom Renderers and Layouts
 
 ```typescript