refactor(utils): add unified b2s/s2b conversion functions

Extract duplicate b2s/s2b functions from proxy/utils.go into
internal/utils/bytes.go. These are zero-allocation unsafe conversions
for byte slice <-> string conversion.

- Create utils.B2s() and utils.S2b() as unified implementations
- Update proxy/utils.go to call utils functions
- Add safety documentation about shared memory warning

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/proxy/utils.go

@@ -2,27 +2,22 @@ package proxy
 
 import (
 	"bytes"
-	"unsafe"
+
+	"rua.plus/lolly/internal/utils"
 )
 
 // b2s converts byte slice to string without allocation.
 // WARNING: The returned string shares memory with the original slice.
 // Do not modify the slice after calling this function.
 func b2s(b []byte) string {
-	if len(b) == 0 {
-		return ""
-	}
-	return unsafe.String(&b[0], len(b))
+	return utils.B2s(b)
 }
 
 // s2b converts string to byte slice without allocation.
 // WARNING: The returned slice shares memory with the original string.
 // Do not modify the slice contents.
 func s2b(s string) []byte {
-	if s == "" {
-		return nil
-	}
-	return unsafe.Slice(unsafe.StringData(s), len(s))
+	return utils.S2b(s)
 }
 
 // isInWhitelist checks if a header key is in the whitelist.

internal/utils/bytes.go

@@ -0,0 +1,23 @@
+package utils
+
+import "unsafe"
+
+// B2s converts byte slice to string without allocation.
+// WARNING: The returned string shares memory with the original slice.
+// Do not modify the slice after calling this function.
+func B2s(b []byte) string {
+	if len(b) == 0 {
+		return ""
+	}
+	return unsafe.String(&b[0], len(b))
+}
+
+// S2b converts string to byte slice without allocation.
+// WARNING: The returned slice shares memory with the original string.
+// Do not modify the slice contents.
+func S2b(s string) []byte {
+	if s == "" {
+		return nil
+	}
+	return unsafe.Slice(unsafe.StringData(s), len(s))
+}