refactor(keys): extract master-key package to top-level keys/

Master-key acquisition and the cross-host dump format are a concern distinct from the raw crypto primitives, so crypto/keyretriever moves to an importable top-level keys/. KeyRetriever→Retriever drops the keys.KeyRetriever stutter.

keys/params.go

@@ -0,0 +1,21 @@
+//go:build darwin || linux
+
+package keys
+
+import (
+	"hash"
+
+	"github.com/moond4rk/hackbrowserdata/crypto"
+)
+
+// pbkdf2Params holds platform-specific PBKDF2 parameters (each platform file defines its own).
+type pbkdf2Params struct {
+	salt       []byte
+	iterations int
+	keySize    int
+	hashFunc   func() hash.Hash
+}
+
+func (p pbkdf2Params) deriveKey(secret []byte) []byte {
+	return crypto.PBKDF2Key(secret, p.salt, p.iterations, p.keySize, p.hashFunc)
+}