The point of the keepalive is to hold the client connection open while the engine
is stuck loading — so 'silent' must NOT fall back to the dead legacy path (which
sends nothing until the engine's first byte and lets short-timeout clients/proxies
disconnect). Now ALL streaming inference goes through the front keepalive path:

  * silent    — SSE comment lines (': …') only: keeps the socket alive, emits no
                chunk/content/status (invisible to event parsers).
  * invisible — empty-content chunk + x_queue_info (default).
  * visible   — visible status text.
  * thinking  — reasoning channel (when mode != silent).

The front stays responsive even when the engine is GIL-blocked, so it can drive
these regardless of engine state.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RdMufYvtTbtGDWsiZVoXce