fix(sub): use configured spiderX instead of always randomizing

applyShareRealityParams and SubJsonService.realityData generated a fresh
random spx on every export, so share links, "export all links", and JSON
subscriptions never matched a spiderX configured on the inbound and two
exports of the same client disagreed with each other. Read the value from
realitySettings.settings like pbk/fp/pqv and keep the random value only as
a fallback when none is configured.

Closes #5718

internal/sub/service.go

@@ -1357,6 +1357,11 @@ func applyShareRealityParams(stream map[string]any, params map[string]string) {
 			}
 		}
 		params["spx"] = "/" + random.Seq(15)
+		if spxValue, ok := searchKey(realitySettings, "spiderX"); ok {
+			if spx, ok := spxValue.(string); ok && len(spx) > 0 {
+				params["spx"] = spx
+			}
+		}
 	}
 }