fix: isolate stdout/stderr calls for Linux concurrency safety

Move fflush/dup/dup2 calls into FlushOutput.swift which imports
the C library directly without Foundation, allowing @preconcurrency
to suppress strict concurrency errors on Linux.

Author: claude

Sources/mtpx/Commands/Transfer.swift

@@ -2,12 +2,6 @@ import ArgumentParser
 import Foundation
 import SwiftMTPAsync
 
-#if canImport(Glibc)
-	@preconcurrency import Glibc
-#elseif canImport(Musl)
-	@preconcurrency import Musl
-#endif
-
 struct Transfer: AsyncParsableCommand {
 	static let configuration = CommandConfiguration(
 		commandName: "transfer",
@@ -244,7 +238,7 @@ struct Transfer: AsyncParsableCommand {
 			if pct != lastPct {
 				lastPct = pct
 				print("\r\(name): \(pct)%", terminator: "    ")
-				fflush(stdout)
+				flushStdout()
 			}
 			return .continue
 		}

Sources/mtpx/Completions.swift

@@ -2,12 +2,6 @@ import ArgumentParser
 import Foundation
 import SwiftMTPAsync
 
-#if canImport(Glibc)
-	@preconcurrency import Glibc
-#elseif canImport(Musl)
-	@preconcurrency import Musl
-#endif
-
 enum Completions {
 	static func remotePath(
 		_ words: [String],
@@ -151,24 +145,6 @@ enum Completions {
 		}
 	}
 
-	private static func muteStderr() -> Int32 {
-		fflush(stderr)
-		let saved = dup(STDERR_FILENO)
-		let devNull = open("/dev/null", O_WRONLY)
-		if devNull >= 0 {
-			dup2(devNull, STDERR_FILENO)
-			close(devNull)
-		}
-		return saved
-	}
-
-	private static func restoreStderr(_ saved: Int32) {
-		guard saved >= 0 else { return }
-		fflush(stderr)
-		dup2(saved, STDERR_FILENO)
-		close(saved)
-	}
-
 	private static func pathCompletions(for input: String) async -> [String] {
 		guard let remote = RemotePath(parsing: input) else { return [] }
 

Sources/mtpx/FlushOutput.swift

@@ -0,0 +1,29 @@
+#if canImport(Darwin)
+	import Darwin
+#elseif canImport(Glibc)
+	@preconcurrency import Glibc
+#elseif canImport(Musl)
+	@preconcurrency import Musl
+#endif
+
+func flushStdout() {
+	fflush(stdout)
+}
+
+func muteStderr() -> Int32 {
+	fflush(stderr)
+	let saved = dup(STDERR_FILENO)
+	let devNull = open("/dev/null", O_WRONLY)
+	if devNull >= 0 {
+		dup2(devNull, STDERR_FILENO)
+		close(devNull)
+	}
+	return saved
+}
+
+func restoreStderr(_ saved: Int32) {
+	guard saved >= 0 else { return }
+	fflush(stderr)
+	dup2(saved, STDERR_FILENO)
+	close(saved)
+}