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/detector.go

@@ -0,0 +1,55 @@
+package engine
+
+import (
+	"regexp"
+	"strings"
+	"time"
+
+	"github.com/salvacybersec/keyhunter/pkg/providers"
+	"github.com/salvacybersec/keyhunter/pkg/types"
+)
+
+// Detect applies provider regex patterns and optional entropy checks to a chunk.
+// It returns all findings from the chunk.
+func Detect(chunk types.Chunk, providerList []providers.Provider) []Finding {
+	var findings []Finding
+	content := string(chunk.Data)
+
+	for _, p := range providerList {
+		for _, pat := range p.Patterns {
+			re, err := regexp.Compile(pat.Regex)
+			if err != nil {
+				continue // invalid regex -- skip silently
+			}
+			matches := re.FindAllString(content, -1)
+			for _, match := range matches {
+				// Apply entropy check if threshold is set
+				if pat.EntropyMin > 0 && Shannon(match) < pat.EntropyMin {
+					continue // too low entropy -- likely a placeholder
+				}
+				line := lineNumber(content, match)
+				findings = append(findings, Finding{
+					ProviderName: p.Name,
+					KeyValue:     match,
+					KeyMasked:    MaskKey(match),
+					Confidence:   pat.Confidence,
+					Source:       chunk.Source,
+					SourceType:   "file",
+					LineNumber:   line,
+					Offset:       chunk.Offset,
+					DetectedAt:   time.Now(),
+				})
+			}
+		}
+	}
+	return findings
+}
+
+// lineNumber returns the 1-based line number where match first appears in content.
+func lineNumber(content, match string) int {
+	idx := strings.Index(content, match)
+	if idx < 0 {
+		return 0
+	}
+	return strings.Count(content[:idx], "\n") + 1
+}