over-engineering audit: remove dead code, speculative config, duplicate queues

- delete embedded community resolver snapshot (6,760 lines) + EmbeddedServers +
  DNS_SERVERS=fresh; curated resolvers are the default. Simplify Warm (no
  sampling).
- one generic internal/sample.Queue[T] replaces three near-identical
  random-sample queues (probe, pipeline, discover).
- delete unreachable index.listOpenSearch (OpenSearch is never listable).
- delete classifyCloudFront (its match is always discarded, !ResourceSpecific).
- delete probe.Probe wrapper + Describe; tests use ProbeDetailed.
- delete store.Pool(), model.SourceDNSAlias, discover.QueueLen.
- drop dead config/no-op knobs: pipeline DirectWorkers, discover MaxPerDomain,
  discover S3/GCS/DO flags, BUCKET_SCAN_DO.
~-7,000 lines (mostly the resolver snapshot), 0 deps removed, tests green.

internal/config/config.go

@@ -43,7 +43,6 @@ type Config struct {
 	BucketScanQPS     int
 	BucketScanWorkers int
 	BucketScanQueue   int
-	BucketScanDO      bool
 
 	MetricsAddr   string
 	UIUser        string
@@ -89,7 +88,6 @@ func Load() (*Config, error) {
 		BucketScanQPS:     envInt("BUCKET_SCAN_QPS", 50),
 		BucketScanWorkers: envInt("BUCKET_SCAN_WORKERS", 16),
 		BucketScanQueue:   envInt("BUCKET_SCAN_QUEUE", 65536),
-		BucketScanDO:      envBool("BUCKET_SCAN_DO", false),
 		MetricsAddr:       envString("METRICS_ADDR", ":9090"),
 		UIUser:            envString("UI_USER", ""),
 		UIPass:            envString("UI_PASS", ""),