doc: Document plain clear-on finish behavior (#1379)

- Clarified the `.plain` progress contract. This also
  documents how `clearOnFinish` interacts with plain
  output. Added test coverage for the current behavior.

Sources/TerminalProgress/ProgressConfig.swift

@@ -62,6 +62,7 @@ public struct ProgressConfig: Sendable {
     /// The theme of the progress bar.
     public let theme: ProgressTheme
     /// The flag indicating whether to clear the progress bar before resetting the cursor.
+    /// In `.plain` mode, `finish()` only emits a final line when this value is `false`.
     public let clearOnFinish: Bool
     /// The flag indicating whether to update the progress bar.
     public let disableProgressUpdates: Bool
@@ -89,7 +90,8 @@ public struct ProgressConfig: Sendable {
     ///   - totalSize: The initial total size of the progress bar. The default value is `nil`.
     ///   - width: The width of the progress bar in characters. The default value is `120`.
     ///   - theme: The theme of the progress bar. The default value is `nil`.
-    ///   - clearOnFinish: The flag indicating whether to clear the progress bar before resetting the cursor. The default is `true`.
+    ///   - clearOnFinish: The flag indicating whether to clear the progress bar before resetting the cursor. In `.plain` mode,
+    ///     `finish()` only emits a final line when this value is `false`. The default is `true`.
     ///   - disableProgressUpdates: The flag indicating whether to update the progress bar. The default is `false`.
     ///   - outputMode: The output mode for progress rendering. The default is `.ansi`.
     public init(

Tests/TerminalProgressTests/ProgressBarTests.swift

@@ -880,7 +880,7 @@ final class ProgressBarTests: XCTestCase {
         try pipe.fileHandleForWriting.close()
 
         let data = pipe.fileHandleForReading.readDataToEndOfFile()
-        let output = String(data: data, encoding: .utf8) ?? ""
+        let output = String(decoding: data, as: UTF8.self)
         let lines = output.components(separatedBy: "\n").filter { !$0.isEmpty }
         // Expect exactly 2 lines: one from render, one from finish
         XCTAssertEqual(lines.count, 2)
@@ -903,7 +903,7 @@ final class ProgressBarTests: XCTestCase {
         try pipe.fileHandleForWriting.close()
 
         let data = pipe.fileHandleForReading.readDataToEndOfFile()
-        let output = String(data: data, encoding: .utf8) ?? ""
+        let output = String(decoding: data, as: UTF8.self)
         XCTAssertFalse(output.contains("\u{001B}"))
     }
 
@@ -922,12 +922,32 @@ final class ProgressBarTests: XCTestCase {
         try pipe.fileHandleForWriting.close()
 
         let data = pipe.fileHandleForReading.readDataToEndOfFile()
-        let output = String(data: data, encoding: .utf8) ?? ""
+        let output = String(decoding: data, as: UTF8.self)
         // Plain mode should use newlines, not carriage returns
         XCTAssertFalse(output.contains("\r"))
         XCTAssertTrue(output.contains("\n"))
     }
 
+    func testPlainModeDefaultClearOnFinishOmitsFinalLine() async throws {
+        let pipe = Pipe()
+        let config = try ProgressConfig(
+            terminal: pipe.fileHandleForWriting,
+            description: "Task",
+            showSpinner: false,
+            outputMode: .plain
+        )
+        let progress = ProgressBar(config: config)
+        progress.render(force: true)
+        progress.finish()
+        try pipe.fileHandleForWriting.close()
+
+        let data = pipe.fileHandleForReading.readDataToEndOfFile()
+        let output = String(decoding: data, as: UTF8.self)
+        let lines = output.components(separatedBy: "\n").filter { !$0.isEmpty }
+        XCTAssertEqual(lines.count, 1)
+        XCTAssertEqual(lines.first, "Task [0s]")
+    }
+
     func testOutputModeDefaultIsAnsi() async throws {
         let config = try ProgressConfig(description: "Task")
         XCTAssertEqual(config.outputMode, .ansi)