feat(07-03): dedup helper for imported findings

- FindingKey: stable SHA-256 over provider+masked+source+line
- Dedup: preserves first-seen order, returns drop count
- 8 unit tests covering stability, field sensitivity, order preservation

pkg/importer/dedup.go

@@ -0,0 +1,42 @@
+package importer
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+
+	"github.com/salvacybersec/keyhunter/pkg/engine"
+)
+
+// FindingKey returns a stable identity hash for a finding based on the
+// provider name, masked key, source path, and line number. This is the
+// dedup identity used by import pipelines so the same underlying secret
+// is not inserted twice when re-importing the same scanner output.
+//
+// Fields outside this tuple (DetectedAt, Confidence, VerifyStatus, ...)
+// intentionally do not contribute to the key: re-running the same import
+// at a later time must collapse onto the original finding.
+func FindingKey(f engine.Finding) string {
+	payload := fmt.Sprintf("%s\x00%s\x00%s\x00%d", f.ProviderName, f.KeyMasked, f.Source, f.LineNumber)
+	sum := sha256.Sum256([]byte(payload))
+	return hex.EncodeToString(sum[:])
+}
+
+// Dedup removes duplicate findings from in-memory slices before insert.
+// Order of first-seen findings is preserved. Returns the deduplicated
+// slice and the number of duplicates dropped.
+func Dedup(in []engine.Finding) ([]engine.Finding, int) {
+	seen := make(map[string]struct{}, len(in))
+	out := make([]engine.Finding, 0, len(in))
+	dropped := 0
+	for _, f := range in {
+		k := FindingKey(f)
+		if _, ok := seen[k]; ok {
+			dropped++
+			continue
+		}
+		seen[k] = struct{}{}
+		out = append(out, f)
+	}
+	return out, dropped
+}