Add cross-compilation, Linux binary build, and systemd service installer

- Dockerfile.cross: builds static x86_64 musl binary from macOS via Docker
- scripts/build-linux.sh: one-command cross-compilation
- scripts/install-service.sh: systemd service with security hardening
- Bump Rust Docker images to 1.86 for edition2024 support

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -1,3 +1,4 @@
 /target
+/dist
 btest_original
 .claude/