[Feature] Diagnostics toggles — dead-letter logging, debug traces, boot config dump

[pathosDev]

Use case

Dead letters are logged without any throttle, debug-level lifecycle/unhandled tracing needs code, and there is no way to dump the resolved config at boot — the single most useful support artifact for a framework whose behavior is layered over HOCON precedence.

Proposed keys

actor-ts {
  log-config-on-start = off
  log-dead-letters = 10                      # count | on | off
  log-dead-letters-during-shutdown = off
  log-dead-letters-suspend-duration = 5m
  debug { unhandled = off, lifecycle = off, event-stream = off }
  cluster.log-info = on
  cluster.debug { verbose-heartbeat-logging = off, verbose-gossip-logging = off }
  sharding.verbose-debug-logging = off
  remote { log-sent-messages = off, log-received-messages = off, log-frame-size-exceeding = off }
}

Acceptance

• Dead-letter logging suspends after the configured count and resumes after the window; log-config-on-start dumps the merged config once.
• Keys ship wired — tests/unit/config/NoDeadConfigKeys.test.ts rejects a key nothing reads.

Part of the reference.conf expansion batch — tracked in #887.