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>

CLAUDE.md

@@ -24,6 +24,12 @@ go vet ./...                            # static analysis
 - **Health checker** — background goroutine pings all nodes on an interval, flips `Node.healthy` atomic bool
 - **RunTx** — panic-safe transaction wrapper: recovers panics, rolls back, re-panics
 - **Querier injection** — `InjectQuerier`/`ExtractQuerier` pass `Querier` via context for service layers
+- **SlogLogger** — adapts `*slog.Logger` to the `dbx.Logger` interface (`slog.go`)
+- **Collect/CollectOne** — generic scan helpers using `pgx.RowToStructByName` (`scan.go`)
+- **Slow query logging** — `Config.SlowQueryThreshold` triggers Warn-level logging in `queryEnd`
+- **PoolStats** — `Cluster.Stats()` aggregates pool statistics across all nodes (`stats.go`)
+- **Tracer passthrough** — `NodeConfig.Tracer` / `WithTracer` sets `pgx.QueryTracer` for OpenTelemetry
+- **RunInTx** — test helper that runs a callback in an always-rolled-back transaction (`dbxtest/tx.go`)
 
 ### Error classification
 
@@ -40,6 +46,7 @@ go vet ./...                            # static analysis
 - `atomic.Bool` for thread safety (`Node.healthy`, `Cluster.closed`)
 - `dbxtest.NewTestCluster` skips tests when DB unreachable, auto-closes via `t.Cleanup`
 - `dbxtest.TestLogger` writes to `testing.T` for test log output
+- `dbxtest.RunInTx` runs a callback in a transaction that is always rolled back
 
 ## See also