test: add obfuscation round-trip tests, fix i18n module import

- 7 passing tests verify client-server compatibility:
  * Handshake obfuscation round-trip (correct key recovers session_id)
  * Wrong key produces garbage (prevents unauthorized probes)
  * Data packet obfuscation round-trip
  * Deterministic derivation (same key = same secrets)
  * Different keys produce different secrets
  * Legacy API consistency
  * Padding range validation (100 random keys)

- Fixed test module import path to use crate::crypto::obfuscation::*
- Added i18n.js module for GUI localization

ostp-core/src/crypto/obfuscation.rs

@@ -198,3 +198,7 @@ pub fn deobfuscate_packet_inplace(raw: &mut [u8], key: &[u8; 8], is_handshake: b
         }
     }
 }
+
+#[cfg(test)]
+#[path = "obfuscation_tests.rs"]
+mod obfuscation_tests;

ostp-core/src/crypto/obfuscation_tests.rs

@@ -1,6 +1,6 @@
 #[cfg(test)]
 mod tests {
-    use super::*;
+    use crate::crypto::obfuscation::*;
 
     /// Verifies that derive_all_secrets is deterministic — same input always
     /// produces the same output.