Tags: rust-proxy/wind

fix(core): allow App to share an externally-owned StatsCollector

The App only created a private StatsCollector when a TrafficSink was
registered, so an application that owns its own collector (e.g. one handed
to a REST API) had no way to make the inbound hooks write into it — the
inbounds saw `InboundHooks.stats == None` and traffic was never recorded.

Add `App::set_stats_collector`, which injects an externally-owned
collector that takes priority over the private one created inside `run`.
The injected instance is used as the inbound hooks' stats handle and, when
a sink is registered, as the flush task's data source, so the App, the
sink, the inbound hooks, and any external reader share one collector.

Add unit tests covering the injected-share and default-disabled cases.

Assisted-by: Reasonix:deepseek-v4-flash