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/format.go

@@ -16,17 +16,17 @@ func printTable(w io.Writer, rows [][]string) {
 	}
 	tw := tabwriter.NewWriter(w, 0, 0, 2, ' ', 0)
 	for i, row := range rows {
-		fmt.Fprintln(tw, strings.Join(row, "\t"))
+		_, _ = fmt.Fprintln(tw, strings.Join(row, "\t"))
 		if i == 0 {
 			// Print a divider matching the header width.
 			dashes := make([]string, len(row))
 			for j, h := range row {
 				dashes[j] = strings.Repeat("-", len(h))
 			}
-			fmt.Fprintln(tw, strings.Join(dashes, "\t"))
+			_, _ = fmt.Fprintln(tw, strings.Join(dashes, "\t"))
 		}
 	}
-	tw.Flush()
+	_ = tw.Flush()
 }
 
 // fmtCount formats a count with a space as the thousands separator.