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/frontend/filter.go

@@ -51,7 +51,7 @@ func applyTerm(term string, fs *filterState) error {
 	// Find the first operator character: ~, !, >, <, =
 	opIdx := strings.IndexAny(term, "~!><=")
 	if opIdx <= 0 {
-		return fmt.Errorf("invalid term %q: expected field=value, field>=value, field~=regex, etc.", term)
+		return fmt.Errorf("invalid term %q: expected field=value, field>=value, field~=regex, etc", term)
 	}
 
 	field := strings.ToLower(strings.TrimSpace(term[:opIdx]))