feat: complete large-book pagination runtime and UI coverage

Sources/RDReaderView/EPUBUI/ReaderController/ChapterRuntime/RDEPUBBackgroundTrace.swift

@@ -0,0 +1,39 @@
+import Foundation
+
+enum RDEPUBBackgroundTrace {
+    static func log(_ scope: String, _ message: String) {
+        let threadRole = Thread.isMainThread ? "main" : "bg"
+        let threadName = resolvedThreadName()
+        let queueLabel = resolvedQueueLabel()
+        print("[EPUB][\(scope)][\(threadRole)][queue=\(queueLabel)][thread=\(threadName)] \(message)")
+    }
+
+    static func measure<T>(_ scope: String, _ message: String, work: () throws -> T) rethrows -> T {
+        let startedAt = CFAbsoluteTimeGetCurrent()
+        log(scope, "START \(message)")
+        do {
+            let result = try work()
+            let elapsedMs = Int((CFAbsoluteTimeGetCurrent() - startedAt) * 1000)
+            log(scope, "END \(message) elapsedMs=\(elapsedMs)")
+            return result
+        } catch {
+            let elapsedMs = Int((CFAbsoluteTimeGetCurrent() - startedAt) * 1000)
+            log(scope, "FAIL \(message) elapsedMs=\(elapsedMs) error=\(error)")
+            throw error
+        }
+    }
+
+    private static func resolvedThreadName() -> String {
+        if let name = Thread.current.name, !name.isEmpty {
+            return name
+        }
+        if Thread.isMainThread {
+            return "main"
+        }
+        return String(describing: Unmanaged.passUnretained(Thread.current).toOpaque())
+    }
+
+    private static func resolvedQueueLabel() -> String {
+        String(validatingUTF8: __dispatch_queue_get_label(nil)) ?? "unknown"
+    }
+}