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/types/chunk.go

@@ -0,0 +1,10 @@
+package types
+
+// Chunk is a segment of file content passed through the scanning pipeline.
+// Defined in pkg/types (not pkg/engine) so that pkg/engine/sources can use it
+// without creating a circular import with pkg/engine.
+type Chunk struct {
+	Data   []byte // raw bytes
+	Source string // file path, URL, or description
+	Offset int64  // byte offset of this chunk within the source
+}