feat(discovery): quiet per-packet logs by default; CLI keeps verbose

Discovery cycles emit one line per UPnP M-SEARCH header, one per
parsed response, and one per enrichment step — by default. A typical
service-binary cycle prints ~50–80 lines for a 3-speaker LAN. Most
operators want a startup-and-summary view; the per-packet trace is
only useful for debugging.

- New SetVerbose/IsVerbose/logVerbose helpers in pkg/discovery (atomic
  bool, zero-value off).
- Chatty log.Printf calls in upnp.go and mdns.go demoted to logVerbose:
  per-header dumps, per-response dumps, per-device enrichment steps,
  M-SEARCH details, read-deadline / cancel-context noise.
- Kept at default level: discovery start ("Starting SSDP discovery
  for…"), end ("Discovery completed. Processed N responses, found N
  unique devices" + per-device summary), warnings ("Configured
  interface not found", "Failed to fetch device description", …), and
  the new "Rejecting non-Bose device" classifier.
- cmd/soundtouch-cli/discover devices grew a --verbose / -v flag that
  flips the package toggle on; the service binary leaves it at the
  zero value.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/soundtouch-cli/cmd_discover.go

@@ -15,6 +15,11 @@ import (
 func discoverDevices(c *cli.Context) error {
 	fmt.Printf("Discovering SoundTouch devices...\n")
 
+	// CLI discovery is interactive — flip on verbose protocol logging
+	// so operators can see per-packet / per-header detail. The service
+	// binary leaves this off so its log stays terse.
+	discovery.SetVerbose(c.Bool("verbose"))
+
 	// Load configuration
 	cfg, err := config.LoadFromEnv()
 	if err != nil {

cmd/soundtouch-cli/main.go

@@ -132,6 +132,11 @@ func main() {
 								Aliases: []string{"a"},
 								Usage:   "Show detailed information for all devices",
 							},
+							&cli.BoolFlag{
+								Name:    "verbose",
+								Aliases: []string{"v"},
+								Usage:   "Print per-packet/per-header SSDP and mDNS trace logs",
+							},
 						},
 					},
 				},