feat(01-04): implement three-stage scanning pipeline with ants worker pool

- pkg/engine/sources/source.go: Source interface using pkg/types.Chunk
- pkg/engine/sources/file.go: FileSource with overlapping chunk reads
- pkg/engine/filter.go: KeywordFilter using Aho-Corasick pre-filter
- pkg/engine/detector.go: Detect with regex matching + Shannon entropy check
- pkg/engine/engine.go: Engine.Scan orchestrating 3-stage pipeline with ants pool
- pkg/engine/scanner_test.go: filled test stubs with pipeline integration tests
- testdata/samples: fixed anthropic key lengths to match {93,} regex pattern

pkg/engine/filter.go

@@ -0,0 +1,17 @@
+package engine
+
+import (
+	ahocorasick "github.com/petar-dambovaliev/aho-corasick"
+	"github.com/salvacybersec/keyhunter/pkg/types"
+)
+
+// KeywordFilter filters a stream of chunks using an Aho-Corasick automaton.
+// Only chunks that contain at least one provider keyword are sent to out.
+// This is Stage 2 of the pipeline (runs after Source, before Detector).
+func KeywordFilter(ac ahocorasick.AhoCorasick, in <-chan types.Chunk, out chan<- types.Chunk) {
+	for chunk := range in {
+		if len(ac.FindAll(string(chunk.Data))) > 0 {
+			out <- chunk
+		}
+	}
+}