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/aggregator/cache.go

@@ -97,15 +97,11 @@ func (c *Cache) LoadHistorical(fine, coarse []st.Snapshot) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
-	for i, snap := range fine {
-		c.fineRing[i] = snap
-	}
+	copy(c.fineRing[:], fine)
 	c.fineFilled = len(fine)
 	c.fineHead = len(fine) % st.FineRingSize
 
-	for i, snap := range coarse {
-		c.coarseRing[i] = snap
-	}
+	copy(c.coarseRing[:], coarse)
 	c.coarseFilled = len(coarse)
 	c.coarseHead = len(coarse) % st.CoarseRingSize
 }