feat(go-services): tracker share_* handlers (complete ingest) + shadow tuning

- share.go: cross-machine vital sharing (share_subscribe/unsubscribe/share_*),
  faithful port of the peer-state snapshot + plugin fan-out + /vital-sharing/peers.
  The last ingest handler — the Go tracker now handles every plugin event type.
- shadow consumer: drop the outbound keepalive ping (the firehose is never idle)
  and tighten the read-deadline watchdog to 12s for faster reconnect after the
  upstream's periodic eviction (full-firehose browser clients get evicted ~every
  90s; the watchdog recovers it, ~90% duty cycle). Production-bound /ws/position
  is unaffected (plugins connect to us; no eviction).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

go-services/tracker-go/shadow.go

@@ -48,22 +48,8 @@ func (s *Server) shadowConnect(ctx context.Context, wsURL string) error {
 
 	connCtx, cancel := context.WithCancel(ctx)
 	defer cancel()
-
-	// Keepalive pings.
-	go func() {
-		t := time.NewTicker(20 * time.Second)
-		defer t.Stop()
-		for {
-			select {
-			case <-connCtx.Done():
-				return
-			case <-t.C:
-				pc, cc := context.WithTimeout(connCtx, 10*time.Second)
-				_ = c.Ping(pc)
-				cc()
-			}
-		}
-	}()
+	// No outbound keepalive ping: the firehose is constant, so the connection is
+	// never idle, and the read-deadline watchdog below handles dead connections.
 
 	// Decouple socket read from ALL processing, including JSON parsing: the read
 	// loop only copies raw frames onto a queue, so it drains the socket as fast
@@ -96,9 +82,9 @@ func (s *Server) shadowConnect(ctx context.Context, wsURL string) error {
 func (s *Server) shadowReadLoop(ctx context.Context, c *websocket.Conn, queue chan []byte, n, dropped *int) error {
 	for {
 		// Read deadline acts as a liveness watchdog: the firehose is constant, so
-		// a long silence means the upstream evicted us without closing — time out
-		// and let runShadowConsumer reconnect.
-		rctx, rcancel := context.WithTimeout(ctx, 25*time.Second)
+		// a multi-second silence means the upstream evicted us without closing —
+		// time out quickly and let runShadowConsumer reconnect (high duty cycle).
+		rctx, rcancel := context.WithTimeout(ctx, 12*time.Second)
 		_, raw, err := c.Read(rctx)
 		rcancel()
 		if err != nil {