Add custom types for type safety throughout codebase

- Add FileCount, FileSize, RelFilePath, AbsFilePath, ModTime, Multihash types
- Add UnixSeconds and UnixNanos types for timestamp handling
- Add URL types (ManifestURL, FileURL, BaseURL) with safe path joining
- Consolidate scanner package into mfer package
- Update checker to use custom types in Result and CheckStatus
- Add ModTime.Timestamp() method for protobuf conversion
- Update all tests to use proper custom types

mfer/builder_test.go

@@ -0,0 +1,104 @@
+package mfer
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNewBuilder(t *testing.T) {
+	b := NewBuilder()
+	assert.NotNil(t, b)
+	assert.Equal(t, 0, b.FileCount())
+}
+
+func TestBuilderAddFile(t *testing.T) {
+	b := NewBuilder()
+	content := []byte("test content")
+	reader := bytes.NewReader(content)
+
+	bytesRead, err := b.AddFile("test.txt", FileSize(len(content)), ModTime(time.Now()), reader, nil)
+	require.NoError(t, err)
+	assert.Equal(t, FileSize(len(content)), bytesRead)
+	assert.Equal(t, 1, b.FileCount())
+}
+
+func TestBuilderAddFileWithHash(t *testing.T) {
+	b := NewBuilder()
+	hash := make([]byte, 34) // SHA256 multihash is 34 bytes
+
+	err := b.AddFileWithHash("test.txt", 100, ModTime(time.Now()), hash)
+	require.NoError(t, err)
+	assert.Equal(t, 1, b.FileCount())
+}
+
+func TestBuilderAddFileWithHashValidation(t *testing.T) {
+	t.Run("empty path", func(t *testing.T) {
+		b := NewBuilder()
+		hash := make([]byte, 34)
+		err := b.AddFileWithHash("", 100, ModTime(time.Now()), hash)
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "path")
+	})
+
+	t.Run("negative size", func(t *testing.T) {
+		b := NewBuilder()
+		hash := make([]byte, 34)
+		err := b.AddFileWithHash("test.txt", -1, ModTime(time.Now()), hash)
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "size")
+	})
+
+	t.Run("nil hash", func(t *testing.T) {
+		b := NewBuilder()
+		err := b.AddFileWithHash("test.txt", 100, ModTime(time.Now()), nil)
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "hash")
+	})
+
+	t.Run("empty hash", func(t *testing.T) {
+		b := NewBuilder()
+		err := b.AddFileWithHash("test.txt", 100, ModTime(time.Now()), []byte{})
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "hash")
+	})
+
+	t.Run("valid inputs", func(t *testing.T) {
+		b := NewBuilder()
+		hash := make([]byte, 34)
+		err := b.AddFileWithHash("test.txt", 100, ModTime(time.Now()), hash)
+		assert.NoError(t, err)
+		assert.Equal(t, 1, b.FileCount())
+	})
+}
+
+func TestBuilderBuild(t *testing.T) {
+	b := NewBuilder()
+	content := []byte("test content")
+	reader := bytes.NewReader(content)
+
+	_, err := b.AddFile("test.txt", FileSize(len(content)), ModTime(time.Now()), reader, nil)
+	require.NoError(t, err)
+
+	var buf bytes.Buffer
+	err = b.Build(&buf)
+	require.NoError(t, err)
+
+	// Should have magic bytes
+	assert.True(t, strings.HasPrefix(buf.String(), MAGIC))
+}
+
+func TestBuilderBuildEmpty(t *testing.T) {
+	b := NewBuilder()
+
+	var buf bytes.Buffer
+	err := b.Build(&buf)
+	require.NoError(t, err)
+
+	// Should still produce valid manifest with 0 files
+	assert.True(t, strings.HasPrefix(buf.String(), MAGIC))
+}