Standalone Go servers report exit codes; vc dev kept alive across requests

Patch Changes

• 94671a4: [go] Supervise the user server after startup in standalone server mode. If the user's server process exits after the server-started handshake, the bootstrap now reports an unrecoverable-error over IPC with the child's exit code, so the platform can recycle the instance instead of leaving it serving 502s while the health check still reports OK.
• d4547af: Fix vc dev for standalone Go server mode:
  • matching the real Go entrypoint when the framework preset's placeholder src does not exist
  • serve all request paths
  • keep a persistent dev server across requests instead of respawning go run per request, and wait for the server port to be ready instead of a fixed 2s window