Add vitepress docs

docs/net/tracker.md

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+# Tracker
+
+The Tracker is a simple but crucial class that keeps track of which relays an event was seen on or published to. It's essential for relay selection and event source tracking.
+
+## Overview
+
+```typescript
+import {Tracker} from '@welshman/net'
+
+const tracker = new Tracker()
+
+// Track event source
+tracker.track(eventId, relayUrl)
+
+// Get relays for event
+const relays = tracker.getRelays(eventId)  // Set<string>
+
+// Get events from relay
+const events = tracker.getIds(relayUrl)    // Set<string>
+
+// Check specific relay
+const seen = tracker.hasRelay(eventId, relayUrl)
+```
+
+## Used By
+
+1. **Repository & Sync**
+```typescript
+// In sync operations
+pull({
+  events,
+  relays,
+  onEvent: (event) => {
+    tracker.track(event.id, relay)
+  }
+})
+```
+
+2. **Subscribe**
+```typescript
+// In @welshman/app subscribe
+sub.on('event', (url, event) => {
+  // Track where we got the event
+  tracker.track(event.id, url)
+})
+```
+
+3. **Publish**
+```typescript
+// In publish operations
+pub.emitter.on('success', (url) => {
+  // Track where we published
+  tracker.track(event.id, url)
+})
+```
+
+4. **Router**
+```typescript
+// Used for relay selection
+const relays = tracker
+  .getRelays(event.id)
+  .filter(url =>
+    isHealthyRelay(url)
+  )
+```
+
+The Tracker:
+- Maps events to their source relays
+- Maps relays to their known events
+- Helps optimize relay selection
+
+Think of it as a memory of where events came from, helping make better decisions about where to find or publish events.