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>

cmd/collector/parser_test.go

@@ -213,9 +213,9 @@ func TestParseLine(t *testing.T) {
 }
 
 func TestParseUDPLine(t *testing.T) {
-	// host \t remote_addr \t method \t uri \t status \t body_bytes \t req_time \t
+	// v1 \t host \t remote_addr \t method \t uri \t status \t body_bytes \t req_time \t
 	// is_tor \t asn \t source_tag \t server_addr \t scheme
-	good := "www.example.com\t1.2.3.4\tGET\t/api/v1/search?q=foo\t200\t1452\t0.043\t0\t12345\tcdn\t10.0.0.1\thttps"
+	good := "v1\twww.example.com\t1.2.3.4\tGET\t/api/v1/search?q=foo\t200\t1452\t0.043\t0\t12345\tcdn\t10.0.0.1\thttps"
 
 	tests := []struct {
 		name   string
@@ -224,7 +224,7 @@ func TestParseUDPLine(t *testing.T) {
 		want   LogRecord
 	}{
 		{
-			name:   "all 12 fields parsed, query stripped, extras dropped",
+			name:   "v1 payload parsed, query stripped, extras dropped",
 			line:   good,
 			wantOK: true,
 			want: LogRecord{
@@ -241,8 +241,8 @@ func TestParseUDPLine(t *testing.T) {
 			},
 		},
 		{
-			name:   "is_tor=1, tag direct, IPv6",
-			line:   "h\t2001:db8::1\tGET\t/\t200\t0\t0\t1\t65535\tdirect\t::1\thttp",
+			name:   "v1 IPv6 tor=1 direct tag",
+			line:   "v1\th\t2001:db8::1\tGET\t/\t200\t0\t0\t1\t65535\tdirect\t::1\thttp",
 			wantOK: true,
 			want: LogRecord{
 				Website:       "h",
@@ -258,18 +258,33 @@ func TestParseUDPLine(t *testing.T) {
 			},
 		},
 		{
-			name:   "11 fields rejected",
-			line:   "h\t1.2.3.4\tGET\t/\t200\t0\t0\t0\t0\ttag\t10.0.0.1",
+			name:   "v1 payload with 11 fields rejected",
+			line:   "v1\th\t1.2.3.4\tGET\t/\t200\t0\t0\t0\t0\ttag\t10.0.0.1",
 			wantOK: false,
 		},
 		{
-			name:   "13 fields rejected",
+			name:   "v1 payload with 13 fields rejected",
 			line:   good + "\textra",
 			wantOK: false,
 		},
 		{
-			name:   "bad IP rejected",
-			line:   "h\tnope\tGET\t/\t200\t0\t0\t0\t0\ttag\t10.0.0.1\thttp",
+			name:   "v1 bad IP rejected",
+			line:   "v1\th\tnope\tGET\t/\t200\t0\t0\t0\t0\ttag\t10.0.0.1\thttp",
+			wantOK: false,
+		},
+		{
+			name:   "unknown version rejected (future v2)",
+			line:   "v2\twww.example.com\t1.2.3.4\tGET\t/\t200\t0\t0\t0\t0\ttag\t10.0.0.1\thttp",
+			wantOK: false,
+		},
+		{
+			name:   "missing version prefix rejected (legacy 12-field line)",
+			line:   "www.example.com\t1.2.3.4\tGET\t/\t200\t0\t0\t0\t0\ttag\t10.0.0.1\thttp",
+			wantOK: false,
+		},
+		{
+			name:   "no tab at all rejected",
+			line:   "v1",
 			wantOK: false,
 		},
 	}