fix: Additional medium-priority bugs and documentation

Bug 7: MAC2 buffer size
- Changed from fixed 1024-byte buffer to dynamic allocation
- Uses malloc/free for packets up to WG_MAX_PACKET_SIZE

Documentation:
- Added comments about WG_TYPE constants sharing values (intentional)
- Added note about platform-specific RNG for embedded systems
- system_rng() uses POSIX /dev/urandom - replace for u-boot/ESP32

Known limitations:
- chain_key initialization uses simplified version (peer_public_key directly)
  Full handshake would use HASH("Noise_IKpsk2_25519...")
- For test phase, simplified version is acceptable

Test results: 29 passed, 3 failed (unchanged)

CMakeLists.txt

@@ -74,3 +74,7 @@ install(FILES include/se050_wireguard.h
 # Install library
 install(TARGETS se050_wireguard
         ARCHIVE DESTINATION lib)
+
+# Note: For embedded platforms (ESP32, u-boot), replace system_rng() with
+# platform-specific RNG (e.g., get_random_bytes() for ESP32)
+# See se050_wireguard.c for details.