feat(dist): add standalone Linux client release package

New client-only distribution: ostp-client-linux-x64.tar.gz (0.92 MB)

Contents:
- ostp-client-linux (2.0 MB) - Universal musl static binary
- README.md (8.3 KB) - Complete user guide with examples
- client.json.example - Configuration template
- SHA256SUMS - Integrity verification

Documentation includes:
- Quick start guide (install, setup, connect)
- All CLI commands with examples
- Profile management workflow
- Configuration options
- Troubleshooting guide
- Security features overview
- Systemd service template
- Advanced usage (scripting, multiple profiles)

Commands:
- ostp-client-linux setup           # Interactive wizard
- ostp-client-linux connect --profile default
- ostp-client-linux status          # Show stats
- ostp-client-linux disconnect
- ostp-client-linux profiles list   # Manage profiles

Features:
- Profile storage: ~/.config/ostp/profiles.json
- TUN interface: ostp0 (10.X.Y.Z/16)
- TLS mimicry with geo-specific SNI
- Anti-VM detection (production only)
- Root privilege check (libc::geteuid)

Distribution strategy:
- Server package: ostp-server-linux-x64.tar.gz (6.86 MB) - server + client + master
- Client package: ostp-client-linux-x64.tar.gz (0.92 MB) - client only
- Windows package: ostp-client-windows-x64.zip (1.29 MB) - GUI + daemon + installer

dist/ostp-client-linux/SHA256SUMS

@@ -0,0 +1 @@
+cf3996eac77ed62d184452b3032e3bffc60c120e77cee57899a33893322b0cc4  ostp-client-linux