feat: M4 gossip ops and UX

Refs #1

- /v1/diff endpoint returns each node's fingerprint set + tombstoned
  auto-thread status
- knox gossip diff <peer-url>: shows peer-only/local-only fingerprints
  (pull/keep preview) and tombstone divergence (resolved-on-peer vs
  would-resurrect)
- knox gossip sync: one-shot anti-entropy sweep + reconcile
- gossip server starts before initial seed so peers can reach a booting
  node; KNOX_PEER_ADDR alone now serves without KNOX_PEERS
- integration tests for diff + tombstone reporting
- e2e verified: two live daemons, diff previewed 1655 peer-only fps,
  sync converged second node to 1655 observations

internal/watch/watch.go

@@ -50,6 +50,21 @@ func (w *Watcher) Start() error {
 	}
 	defer watcher.Close()
 
+	// Start the gossip server first so peers can reach us while the initial
+	// seed is still ingesting. Vault/dirs are logged after the seed below.
+	gossipAddr := ListenAddr()
+	node := &Node{Kdb: w.knoxDB, Name: "knox", Addr: gossipAddr}
+	srv := &http.Server{Addr: gossipAddr, Handler: node.Handler()}
+	go func() {
+		log.Printf("[knox] gossip listening on %s", gossipAddr)
+		if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+			log.Printf("[knox] gossip server: %v", err)
+		}
+	}()
+	if peers := PeerAddrs(); len(peers) > 0 {
+		log.Printf("[knox] gossip peers: %v", peers)
+	}
+
 	w.seed(watcher)
 	log.Printf("[knox] watching %d directories", len(w.dirs))
 
@@ -57,20 +72,6 @@ func (w *Watcher) Start() error {
 		log.Printf("[knox] obsidian vault: %s", w.vault)
 	}
 
-	// Gossip: serve our observation log to peers and periodically converge.
-	gossipAddr := ListenAddr()
-	if peers := PeerAddrs(); len(peers) > 0 {
-		node := &Node{Kdb: w.knoxDB, Name: "knox", Addr: gossipAddr}
-		srv := &http.Server{Addr: gossipAddr, Handler: node.Handler()}
-		go func() {
-			log.Printf("[knox] gossip listening on %s", gossipAddr)
-			if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
-				log.Printf("[knox] gossip server: %v", err)
-			}
-		}()
-		log.Printf("[knox] gossip peers: %v", peers)
-	}
-
 	debounceMap := make(map[string]time.Time)
 	browserTicker := time.NewTicker(browserInterval)
 	giteaTicker := time.NewTicker(10 * time.Minute)