fix(zapio): correct carriage return handling per review feedback

Based on reviewer feedback, the carriage return (\r) handling has been corrected:

- Standalone \r now discards any buffered content without logging, matching the intent of issue #1055.
- This prevents logging duplicate lines for progress bars (e.g., git clone progress).
- Only \n and \r\n sequences trigger actual log messages.
- \r\n is correctly handled as a single separator (Windows line ending).

Test cases have been updated to reflect the correct behavior:
- "carriage return_clears_buffer" tests buffer discard without logging
- "progress-style_output" now tests only the final message is logged
- "carriage_return_with_buffered_content" tests buffer discard correctly

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

Author: lyydsheep

zapio/writer.go

@@ -90,6 +90,8 @@ func (w *Writer) Write(bs []byte) (n int, err error) {
 // unconsumed bytes.
 //
 // It handles both newlines (\n) and carriage returns (\r).
+// \n and \r\n cause the buffer to be flushed to the logger.
+// Standalone \r discards any buffered content without logging (for overwriting progress).
 func (w *Writer) writeLine(line []byte) (remaining []byte) {
 	// Find the first occurrence of either \n or \r
 	nlIdx := bytes.IndexByte(line, '\n')
@@ -98,6 +100,7 @@ func (w *Writer) writeLine(line []byte) (remaining []byte) {
 	// Determine which separator comes first (or if neither exists)
 	sepIdx := -1
 	sepLen := 0
+	crOnly := false
 
 	if nlIdx >= 0 && (crIdx < 0 || nlIdx <= crIdx) {
 		sepIdx = nlIdx
@@ -109,6 +112,7 @@ func (w *Writer) writeLine(line []byte) (remaining []byte) {
 			sepLen = 2
 		} else {
 			sepLen = 1
+			crOnly = true
 		}
 	}
 
@@ -121,6 +125,13 @@ func (w *Writer) writeLine(line []byte) (remaining []byte) {
 	// Split on the separator, buffer and flush the left.
 	line, remaining = line[:sepIdx], line[sepIdx+sepLen:]
 
+	if crOnly {
+		// A standalone \r discards any buffered content without logging.
+		// This handles progress bars that overwrite themselves.
+		w.buff.Reset()
+		return remaining
+	}
+
 	// Fast path: if we don't have a partial message from a previous write
 	// in the buffer, skip the buffer and log directly.
 	if w.buff.Len() == 0 {

zapio/writer_test.go

@@ -127,12 +127,11 @@ func TestWriter(t *testing.T) {
 			},
 		},
 		{
-			desc: "carriage return creates line break",
+			desc: "carriage return clears buffer",
 			writes: []string{
-				"foo\rbar\r",
+				"foo\rbar",
 			},
 			want: []zapcore.Entry{
-				{Level: zap.InfoLevel, Message: "foo"},
 				{Level: zap.InfoLevel, Message: "bar"},
 			},
 		},
@@ -149,14 +148,47 @@ func TestWriter(t *testing.T) {
 		{
 			desc: "progress-style output with multiple updates",
 			writes: []string{
-				"progress: 10%\rprogress: 25%\rprogress: 50%\r",
+				"progress: 10%\rprogress: 25%\rprogress: 50%\r\n",
 			},
 			want: []zapcore.Entry{
-				{Level: zap.InfoLevel, Message: "progress: 10%"},
-				{Level: zap.InfoLevel, Message: "progress: 25%"},
 				{Level: zap.InfoLevel, Message: "progress: 50%"},
 			},
 		},
+		{
+			desc: "mixed newlines and carriage returns",
+			writes: []string{
+				"foo\nbar\r\rbaz\r\nqux\n",
+			},
+			want: []zapcore.Entry{
+				{Level: zap.InfoLevel, Message: "foo"},
+				{Level: zap.InfoLevel, Message: "baz"},
+				{Level: zap.InfoLevel, Message: "qux"},
+			},
+		},
+		{
+			desc: "carriage return with buffered content",
+			writes: []string{
+				"foo",
+				"ba",
+				"r\rqux",
+				"\n",
+			},
+			want: []zapcore.Entry{
+				{Level: zap.InfoLevel, Message: "qux"},
+			},
+		},
+		{
+			desc: "carriage return newline with buffered content",
+			writes: []string{
+				"foo",
+				"ba",
+				"r\r\nqux\r\n",
+			},
+			want: []zapcore.Entry{
+				{Level: zap.InfoLevel, Message: "foobar"},
+				{Level: zap.InfoLevel, Message: "qux"},
+			},
+		},
 	}
 
 	for _, tt := range tests {