RELEASE 1.0.1: v2 log format, source_tag-labeled metrics, lint cleanup

Wire-format and metric overhaul. Both file and UDP ingest now share one
versioned ParseLine that dispatches on the v<N>\t prefix; v1 stays
unchanged, v2 adds $bytes_sent (replacing $body_bytes_sent),
$request_length, $upstream_response_time, and $upstream_status. File
ingest gains the same versioning, and the legacy positional file format
is removed (no live deployments).

Prometheus exposition is rewritten:

  - nginx_http_bytes_sent and nginx_http_request_duration_seconds gain
    a source_tag label.
  - nginx_http_requests_by_source_total gains status_class.
  - New v2-only metrics: nginx_http_request_bytes,
    nginx_http_upstream_duration_seconds,
    nginx_http_upstream_requests_total{status_class}.
  - Dropped nginx_http_response_body_bytes_by_source (subsumed by the
    dual-labeled bytes_sent metric).

Adds 'make fixstyle' (gofmt -w) and clears all golangci-lint findings
across the repo (errcheck, S1001, ST1005, unused).

Docs in design.md FR-2/FR-8 and user-guide.md are rewritten to present
v2 as the recommended log format.

cmd/cli/cmd_topn.go

@@ -24,7 +24,7 @@ func runTopN(args []string) {
 	n := fs.Int("n", 10, "number of entries")
 	window := fs.String("window", "5m", "time window: 1m 5m 15m 60m 6h 24h")
 	groupBy := fs.String("group-by", "website", "group by: website prefix uri status")
-	fs.Parse(args)
+	_ = fs.Parse(args) // ExitOnError: only returns nil here
 	sf.resolve(*targetFlag)
 
 	win := parseWindow(*window)
@@ -66,7 +66,7 @@ func fanOutTopN(targets []string, filter *pb.Filter, groupBy pb.GroupBy, n int,
 				results[i].resp = &pb.TopNResponse{}
 				return
 			}
-			defer conn.Close()
+			defer func() { _ = conn.Close() }()
 			ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 			defer cancel()
 			resp, err := client.TopN(ctx, &pb.TopNRequest{