feat: dynamically discover external rollout engines

[Aphoh]

Summary

• add --rollout-external-dynamic-discovery-path for synchronous worker control-URL discovery
• refresh membership immediately before weight updates and recreate control actors only when topology changes
• reconnect NCCL weight-update groups using the new membership while disconnecting the prior actors
• retain replaced control actors until the update finishes, then retire them
• require a shared rollout frontend so request routing remains owned by the external system

Dependency

Depends on #2 and has been rebased onto its latest head. Both PRs target main as requested, so this PR temporarily includes #2 in its GitHub diff. Once #2 merges, this PR reduces to its single dynamic-discovery commit.

Validation

• tests/test_external_sglang_engines.py: 11 passed
• all 56 affected discovery, streaming, argument-validation, and custom-generate contract tests passed together after the rebase
• uvx prek run --all-files: passed
• the combined implementation was previously exercised end-to-end with Dynamo-managed workers and NCCL weight updates