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.go

@@ -0,0 +1,23 @@
+package engine
+
+import "math"
+
+// Shannon computes the Shannon entropy of a string in bits per character.
+// Returns 0.0 for empty strings.
+// A value >= 3.5 indicates high randomness, consistent with real API keys.
+func Shannon(s string) float64 {
+	if len(s) == 0 {
+		return 0.0
+	}
+	freq := make(map[rune]float64)
+	for _, c := range s {
+		freq[c]++
+	}
+	n := float64(len([]rune(s)))
+	var entropy float64
+	for _, count := range freq {
+		p := count / n
+		entropy -= p * math.Log2(p)
+	}
+	return entropy
+}

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"))
+}

pkg/engine/finding.go

@@ -0,0 +1,26 @@
+package engine
+
+import "time"
+
+// Finding represents a detected API key from the scanning pipeline.
+// KeyValue holds the plaintext key -- the storage layer encrypts it before persisting.
+type Finding struct {
+	ProviderName string
+	KeyValue     string // full plaintext key
+	KeyMasked    string // first8...last4
+	Confidence   string // "high", "medium", "low"
+	Source       string // file path or description
+	SourceType   string // "file", "dir", "git", "stdin", "url"
+	LineNumber   int
+	Offset       int64
+	DetectedAt   time.Time
+}
+
+// MaskKey returns a masked representation: first 8 chars + "..." + last 4 chars.
+// Returns "****" if the key is shorter than 12 characters.
+func MaskKey(key string) string {
+	if len(key) < 12 {
+		return "****"
+	}
+	return key[:8] + "..." + key[len(key)-4:]
+}