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>

scripts/build-linux.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+# Build a static x86_64 Linux binary using Docker (works from macOS)
+set -euo pipefail
+
+cd "$(dirname "$0")/.."
+
+echo "=== Building x86_64 Linux binary via Docker ==="
+DOCKER_BUILDKIT=1 docker build \
+    -f Dockerfile.cross \
+    --output type=local,dest=./dist \
+    .
+
+ls -lh dist/btest
+file dist/btest
+echo ""
+echo "Binary ready at: dist/btest"
+echo "Copy to your server: scp dist/btest user@server:/usr/local/bin/btest"