fix(client): replace todo!() with stubs to prevent crashes

Problem:
Client terminal was closing immediately on connect command because
todo!() macros in TUN interface and relay functions were causing panics.

Changes:
1. ✅ Replaced create_tun_interface() todo with stub returning dummy interface
2. ✅ Replaced configure_routing() todo with stub (no-op)
3. ✅ Replaced cleanup_routing() todo with stub (no-op)
4. ✅ Replaced relay_traffic() todo with infinite sleep (keeps connection alive)
5. ✅ Added warning messages with yellow ⚠ indicator for stub functions
6. ✅ Updated SHA256SUMS for both packages
7. ✅ Recreated distribution archives
8. ✅ Added CONFIG_FILES.md explaining server-enrollment.json.example usage

Client Behavior Now:
- Connection establishes successfully
- Shows [STUB] warnings for TUN/routing/relay
- Stays connected (Ctrl+C to exit)
- No actual traffic forwarding yet (TODO for next iteration)

server-enrollment.json.example Usage:
- For Standalone mode (connecting to existing Master Node)
- Requires enrollment_token from admin
- psk: 'AUTO' until approved
- See CONFIG_FILES.md for detailed workflow

Next Steps:
- Implement real TUN interface using osn crate
- Implement route configuration via ip command
- Implement packet relay loop (TUN ↔ OSTP client)
- Add daemon mode (fork + detach)

Distribution:
- ostp-server-linux-x64.tar.gz: 6.85 MB
- ostp-client-linux-x64.tar.gz: 0.92 MB (updated client)

dist/ostp-client-linux/SHA256SUMS

@@ -1 +1 @@
-cf3996eac77ed62d184452b3032e3bffc60c120e77cee57899a33893322b0cc4  ostp-client-linux
+18401af97204e60da7cbef94a7f3fab8f77878d95b7dd6da3905e7e5cb505186  ostp-client-linux