coverage: make targets + close zero-cov gaps (namegen, sessionstream)

Adds `make coverage` (per-package + total via -coverpkg=./...),
`make coverage-html`, and `make coverage-total` (CI-friendly). Wires
coverage.out/coverage.html through `make clean` and .gitignore.

Closes the two easy zero-coverage packages: namegen (77.8%) and
sessionstream (93.5%). Total statement coverage 51.7% -> 52.1%.

internal/sessionstream/sessionstream_test.go

@@ -0,0 +1,117 @@
+package sessionstream
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"testing"
+)
+
+func TestWriteReadFrameRoundtrip(t *testing.T) {
+	cases := []struct {
+		name    string
+		channel byte
+		payload []byte
+	}{
+		{"stdout_bytes", ChannelStdout, []byte("hello world")},
+		{"stderr_bytes", ChannelStderr, []byte{0x00, 0xff, 0x7f}},
+		{"empty_payload", ChannelStdin, nil},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			var buf bytes.Buffer
+			if err := WriteFrame(&buf, tc.channel, tc.payload); err != nil {
+				t.Fatalf("WriteFrame: %v", err)
+			}
+			ch, got, err := ReadFrame(&buf)
+			if err != nil {
+				t.Fatalf("ReadFrame: %v", err)
+			}
+			if ch != tc.channel {
+				t.Fatalf("channel = %d, want %d", ch, tc.channel)
+			}
+			if !bytes.Equal(got, tc.payload) && !(len(got) == 0 && len(tc.payload) == 0) {
+				t.Fatalf("payload = %q, want %q", got, tc.payload)
+			}
+		})
+	}
+}
+
+type shortWriter struct {
+	failAfter int
+	written   int
+}
+
+func (s *shortWriter) Write(p []byte) (int, error) {
+	s.written += len(p)
+	if s.written > s.failAfter {
+		return 0, io.ErrShortWrite
+	}
+	return len(p), nil
+}
+
+func TestWriteFrameWriterError(t *testing.T) {
+	w := &shortWriter{failAfter: 2}
+	err := WriteFrame(w, ChannelStdout, []byte("payload"))
+	if err == nil {
+		t.Fatal("expected error from short writer")
+	}
+}
+
+func TestReadFrameTruncated(t *testing.T) {
+	_, _, err := ReadFrame(bytes.NewReader([]byte{0x02, 0x00}))
+	if !errors.Is(err, io.ErrUnexpectedEOF) && err == nil {
+		t.Fatalf("expected EOF-ish error, got %v", err)
+	}
+
+	// Header OK, but payload truncated.
+	var buf bytes.Buffer
+	buf.Write([]byte{ChannelStdout, 0x00, 0x00, 0x00, 0x05})
+	buf.Write([]byte("ab"))
+	if _, _, err := ReadFrame(&buf); err == nil {
+		t.Fatal("expected truncated payload error")
+	}
+}
+
+func TestControlRoundtrip(t *testing.T) {
+	code := 42
+	msg := ControlMessage{Type: "exit", ExitCode: &code}
+
+	var buf bytes.Buffer
+	if err := WriteControl(&buf, msg); err != nil {
+		t.Fatalf("WriteControl: %v", err)
+	}
+
+	got, err := ReadNextControl(&buf)
+	if err != nil {
+		t.Fatalf("ReadNextControl: %v", err)
+	}
+	if got.Type != "exit" {
+		t.Fatalf("type = %q, want exit", got.Type)
+	}
+	if got.ExitCode == nil || *got.ExitCode != 42 {
+		t.Fatalf("exit_code = %v, want 42", got.ExitCode)
+	}
+}
+
+func TestReadControlBadJSON(t *testing.T) {
+	if _, err := ReadControl([]byte("{not json")); err == nil {
+		t.Fatal("expected JSON error")
+	}
+}
+
+func TestReadNextControlWrongChannel(t *testing.T) {
+	var buf bytes.Buffer
+	if err := WriteFrame(&buf, ChannelStdout, []byte("not a control frame")); err != nil {
+		t.Fatalf("WriteFrame: %v", err)
+	}
+	if _, err := ReadNextControl(&buf); err == nil {
+		t.Fatal("expected error for non-control channel")
+	}
+}
+
+func TestFormatConstant(t *testing.T) {
+	if FormatV1 != "stdio_mux_v1" {
+		t.Fatalf("FormatV1 = %q, want stdio_mux_v1", FormatV1)
+	}
+}