feat(01-04): add shared Chunk type, Finding struct, Shannon entropy, and MaskKey

- pkg/types/chunk.go: shared Chunk struct breaking engine<->sources circular import
- pkg/engine/finding.go: Finding struct with MaskKey for pipeline output
- pkg/engine/entropy.go: Shannon entropy function using math.Log2
- pkg/engine/entropy_test.go: TDD tests for Shannon and MaskKey

pkg/engine/entropy_test.go

@@ -0,0 +1,31 @@
+package engine
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestShannonAllSame(t *testing.T) {
+	assert.InDelta(t, 0.0, Shannon("aaaaaaa"), 0.01)
+}
+
+func TestShannonDistinct(t *testing.T) {
+	assert.InDelta(t, 3.0, Shannon("abcdefgh"), 0.1)
+}
+
+func TestShannonRealKey(t *testing.T) {
+	assert.GreaterOrEqual(t, Shannon("sk-proj-ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqr"), 3.5)
+}
+
+func TestShannonEmpty(t *testing.T) {
+	assert.Equal(t, 0.0, Shannon(""))
+}
+
+func TestMaskKeyNormal(t *testing.T) {
+	assert.Equal(t, "sk-proj-...1234", MaskKey("sk-proj-abc1234"))
+}
+
+func TestMaskKeyShort(t *testing.T) {
+	assert.Equal(t, "****", MaskKey("abc"))
+}