PRE-RELEASE 0.9.1: Makefile, Debian packaging, versioned UDP

Build and release tooling:
- Makefile with help as default; targets: build/build-amd64/build-arm64,
  test, lint, proto, pkg-deb, docker, docker-push, clean, plus
  install-deps (+ three sub-targets for apt / Go toolchain / Go tools).
- internal/version package; -ldflags -X injects Version/Commit/Date into
  every binary. -version flag on all four binaries (nginx-logtail version
  for the CLI).
- Dockerfile takes VERSION/COMMIT/DATE build-args and forwards them.
- .deb output lands in build/; .gitignore ignores /build/.

Debian package:
- debian/build-deb.sh packages all four static binaries into a single
  nginx-logtail_<ver>_<arch>.deb using dpkg-deb.
- Binary layout: /usr/sbin/nginx-logtail-{collector,aggregator,frontend}
  and /usr/bin/nginx-logtail.
- nginx-logtail(8) manpage.
- Three systemd units (collector, aggregator, frontend) shipped under
  /lib/systemd/system/. Installed but never enabled or started — the
  operator opts in per host.
- Collector runs as _logtail:www-data (log access); aggregator and
  frontend as _logtail:_logtail. postinst creates the system user/group
  idempotently.
- Single shared env file /etc/default/nginx-logtail rendered from a
  template at first install with %HOSTNAME% substituted. Sensible
  defaults for every COLLECTOR_*, AGGREGATOR_*, FRONTEND_* variable;
  plus COLLECTOR_ARGS / AGGREGATOR_ARGS / FRONTEND_ARGS escape hatches
  appended to ExecStart. Not a dpkg conffile: operator edits survive
  upgrades and dpkg --purge removes it.

Versioned UDP wire format:
- ParseUDPLine dispatches on a leading "v<N>\t" tag; v1 routes to the
  existing 12-field parser. Unknown/missing versions fail closed so
  future v2 parsers can land before emitters are upgraded.
- Tests updated; design.md FR-2.2 rewritten to make the version tag
  normative.

Docs:
- README.md gains a Quick Start (Debian / Docker Compose / from source).
- user-guide.md rewritten around Installation and Configuration: full
  env-var table, UDP-only default explained, precise file/UDP log_format
  layouts, note that operators can emit "0" for unknown \$is_tor / \$asn.
- Drilldown cycle, frontend filter table, and CLI --group-by list all
  include source_tag. UDP counters documented in the Prometheus section.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

debian/nginx-logtail-aggregator.service

@@ -0,0 +1,23 @@
+[Unit]
+Description=nginx-logtail aggregator
+Documentation=man:nginx-logtail(8)
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+User=_logtail
+Group=_logtail
+EnvironmentFile=-/etc/default/nginx-logtail
+ExecStart=/usr/sbin/nginx-logtail-aggregator $AGGREGATOR_ARGS
+Restart=on-failure
+RestartSec=5
+
+# Aggregator needs no filesystem access beyond its binary.
+ProtectSystem=strict
+ProtectHome=yes
+PrivateTmp=yes
+NoNewPrivileges=yes
+
+[Install]
+WantedBy=multi-user.target