Add production features: slog adapter, scan helpers, slow query logging, pool stats, tracer passthrough, test tx isolation

- slog.go: SlogLogger adapts *slog.Logger to dbx.Logger interface
- scan.go: Collect[T] and CollectOne[T] generic helpers using pgx.RowToStructByName
- cluster.go: slow query logging via Config.SlowQueryThreshold (Warn level in queryEnd)
- stats.go: PoolStats with Cluster.Stats() aggregating pool stats across all nodes
- config.go/node.go: NodeConfig.Tracer passthrough for pgx.QueryTracer (OpenTelemetry)
- options.go: WithSlowQueryThreshold and WithTracer functional options
- dbxtest/tx.go: RunInTx runs callback in always-rolled-back transaction for test isolation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

AGENTS.md

@@ -11,17 +11,21 @@ Universal guide for AI coding agents working with this codebase.
 ```
 dbx/                      Root — Cluster, Node, Balancer, retry, health, errors, tx, config
 ├── dbx.go                Interfaces: Querier, DB, Logger, MetricsHook
-├── cluster.go            Cluster — routing, write/read operations
-├── node.go               Node — pgxpool.Pool wrapper with health state
+├── cluster.go            Cluster — routing, write/read operations, slow query logging
+├── node.go               Node — pgxpool.Pool wrapper with health state, tracer passthrough
 ├── balancer.go           Balancer interface + RoundRobinBalancer
 ├── retry.go              retrier — exponential backoff with jitter and node fallback
 ├── health.go             healthChecker — background goroutine pinging nodes
 ├── tx.go                 RunTx, RunTxOptions, InjectQuerier, ExtractQuerier
 ├── errors.go             Error classification (IsRetryable, IsConnectionError, etc.)
 ├── config.go             Config, NodeConfig, PoolConfig, RetryConfig, HealthCheckConfig
-├── options.go            Functional options (WithLogger, WithMetrics, WithRetry, etc.)
+├── options.go            Functional options (WithLogger, WithMetrics, WithRetry, WithTracer, etc.)
+├── slog.go               SlogLogger — adapts *slog.Logger to dbx.Logger
+├── scan.go               Collect[T], CollectOne[T] — generic row scan helpers
+├── stats.go              PoolStats — aggregate pool statistics via Cluster.Stats()
 └── dbxtest/
-    └── dbxtest.go        Test helpers: NewTestCluster, TestLogger
+    ├── dbxtest.go        Test helpers: NewTestCluster, TestLogger
+    └── tx.go             RunInTx — test transaction isolation (always rolled back)
 ```
 
 ## Routing architecture