Add Debian/Ubuntu .deb packaging and update README

Debian packaging via cargo-deb:
- Add [package.metadata.deb] to Cargo.toml with assets, dependencies,
  and conffiles declarations
- Maintainer scripts: postinst (create fips group, enable services,
  restart on upgrade), prerm (stop/disable on remove, stop-only on
  upgrade), postrm (purge config, keys, group)
- Systemd units (fips.service, fips-dns.service) with /usr/bin/ paths
  for .deb installs
- tmpfiles.d entry for /run/fips/ runtime directory
- build-deb.sh wrapper script for building packages

README rewrite:
- Replace run-from-source Quick Start with Building section
- Add Installation options: Debian .deb (cargo-deb) and generic Linux
  (systemd tarball)
- Add full Configuration reference with default fips.yaml
- Add Usage sections: DNS resolution, monitoring (fipsctl + fipstop),
  service management, and testing
- Update project structure and features list

packaging/debian/build-deb.sh

@@ -0,0 +1,41 @@
+#!/usr/bin/env bash
+# Build a .deb package for FIPS using cargo-deb.
+#
+# Usage: ./build-deb.sh
+#
+# Prerequisites: cargo-deb (install with: cargo install cargo-deb)
+# Output: deploy/fips_<version>_<arch>.deb
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+PROJECT_ROOT="${SCRIPT_DIR}/../.."
+
+cd "${PROJECT_ROOT}"
+
+# Ensure cargo-deb is available
+if ! command -v cargo-deb &>/dev/null; then
+    echo "cargo-deb not found. Install with: cargo install cargo-deb" >&2
+    exit 1
+fi
+
+# Build the .deb package
+echo "Building .deb package..."
+cargo deb
+
+# Move output to deploy/
+mkdir -p deploy
+DEB_FILE=$(find target/debian -name '*.deb' -printf '%T@ %p\n' | sort -rn | head -1 | cut -d' ' -f2)
+
+if [ -z "${DEB_FILE}" ]; then
+    echo "Error: No .deb file found in target/debian/" >&2
+    exit 1
+fi
+
+cp "${DEB_FILE}" deploy/
+BASENAME=$(basename "${DEB_FILE}")
+echo "Package built: deploy/${BASENAME}"
+echo ""
+echo "Install with: sudo dpkg -i deploy/${BASENAME}"
+echo "Remove with:  sudo dpkg -r fips"
+echo "Purge with:   sudo dpkg -P fips  (removes config and identity keys)"