Add unit tests for checker/scanner and validate input paths

- Add comprehensive unit tests for internal/checker (88.5% coverage)
  - Status string representations
  - NewChecker validation
  - Check operation (OK, missing, size/hash mismatch)
  - Progress reporting and context cancellation
  - FindExtraFiles functionality

- Add comprehensive unit tests for internal/scanner (80.1% coverage)
  - Constructors and options
  - File/path enumeration
  - Dotfile exclusion/inclusion
  - ToManifest with progress and cancellation
  - Non-blocking status channel sends

- Validate input paths before scanning in generate command
  - Fail fast with clear error if paths don't exist
  - Prevents confusing errors deep in enumeration

internal/cli/entry_test.go

@@ -485,6 +485,42 @@ func TestGenerateAtomicWriteCleansUpOnError(t *testing.T) {
 	assert.False(t, tmpExists, "temp file should be cleaned up after failed generation")
 }
 
+func TestGenerateValidatesInputPaths(t *testing.T) {
+	fs := afero.NewMemMapFs()
+
+	// Create one valid directory
+	require.NoError(t, fs.MkdirAll("/validdir", 0755))
+	require.NoError(t, afero.WriteFile(fs, "/validdir/file.txt", []byte("content"), 0644))
+
+	t.Run("nonexistent path fails fast", func(t *testing.T) {
+		opts := testOpts([]string{"mfer", "generate", "-q", "-o", "/output.mf", "/nonexistent"}, fs)
+		exitCode := RunWithOptions(opts)
+		assert.Equal(t, 1, exitCode)
+		stderr := opts.Stderr.(*bytes.Buffer).String()
+		assert.Contains(t, stderr, "path does not exist")
+		assert.Contains(t, stderr, "/nonexistent")
+	})
+
+	t.Run("mix of valid and invalid paths fails fast", func(t *testing.T) {
+		opts := testOpts([]string{"mfer", "generate", "-q", "-o", "/output.mf", "/validdir", "/alsononexistent"}, fs)
+		exitCode := RunWithOptions(opts)
+		assert.Equal(t, 1, exitCode)
+		stderr := opts.Stderr.(*bytes.Buffer).String()
+		assert.Contains(t, stderr, "path does not exist")
+		assert.Contains(t, stderr, "/alsononexistent")
+
+		// Output file should not have been created
+		exists, _ := afero.Exists(fs, "/output.mf")
+		assert.False(t, exists, "output file should not exist when path validation fails")
+	})
+
+	t.Run("valid paths succeed", func(t *testing.T) {
+		opts := testOpts([]string{"mfer", "generate", "-q", "-o", "/output.mf", "/validdir"}, fs)
+		exitCode := RunWithOptions(opts)
+		assert.Equal(t, 0, exitCode)
+	})
+}
+
 func TestCheckDetectsManifestCorruption(t *testing.T) {
 	fs := afero.NewMemMapFs()
 	rng := rand.New(rand.NewSource(42))