protocol, clients: acks, queue notices and reconnect without duplicates ...

- client_id is echoed in message_accepted / message_queued; queue overflow
  broadcasts message_dropped to every socket watching the session
- stream_end / stream_stopped carry message_index, the global display rank
  that also stamps history rows
- webclient: cheap session_sync via /sessions/{id}/meta, the ack deadline is
  armed at the write and re-armed while a run is active, fatal 4003/4004 stop
  the reconnect loop instead of retrying forever
- TUI: WsBridge posts WsResumed / MessageWritten / SessionUnavailable, the app
  drops the half-written turn on reconnect, shows queued and not-delivered
  markers on the bubble, and reloads history only when the meta token changes
- dist rebuilt

core: split the agent and context builder into packages ...

Agent becomes one class assembled from mixins: runtime (turn lifecycle),
streaming (the turn loop), tools (tool execution), compression (context
gates) and context (tool lists, backend lookup, window). The module-level
helpers and sentinels move to helpers.py.

ContextBuilder splits into helpers (pure truncation/freshness/prompt
blocks) and builder (the class). Both old modules become packages with
the same import surface, so navi.core.agent.Agent and
navi.core.context_builder.ContextBuilder keep working.

Method bodies are unchanged; tests now patch settings/log/should_compress
on the mixin that actually reads them.