Convert the concrete *mux.Session into a mux.Session interface backed by
smux and yamux, selected via the `mux.type` metadata key. Empty/unspecified
defaults to "smux", so existing deployments are unchanged. Yamux's stream
window is exposed via `mux.maxStreamWindow`.
Also fix TestHandler_observeStats: the retry_after_observer_error subtest
hung because stats.Events() drains (IsUpdated is Swap(false)) and no fresh
event was generated for the recovery tick's fall-through flush. Inject a real
client2 event so the recovery tick has new data to flush.
Plan: docs/plans/2026-07-13-mux-pluggable-yamux.md
Translate all Chinese comments in the relay handler package to English:
handler.go, connect.go, bind.go, forward.go, conn.go, entrypoint.go,
observe.go, metadata.go. All inline and doc comments are now in English.
Also set a persistent preference: all future code comments must be
written in English only.
ginuerzh