docs(handler/tunnel): add architecture documentation and fix observeStats event loss

- Add package-level architecture doc in handler.go covering:
  - NAT traversal reverse proxy architecture
  - CmdBind/CmdConnect roles and data flow
  - Connector lifecycle and waitClose semantics
  - Entrypoint protocol dispatch (first-byte sniffing)
  - SD fallback behavior
- Add data-flow-oriented doc comments to all business files:
  - entrypoint.go: protocol dispatch, dial flow
  - connector.go: Connector/ConnectorPool semantics
  - dialer.go: two-phase dial strategy
  - tunnel.go: MaxWeight semantics, selection algorithm
  - bind.go: 6-step CmdBind flow
  - connect.go: CmdConnect flow with relay framing
  - ephttp.go, eptls.go, eprelay.go: per-protocol entrypoint flow
- Fix observeStats: after successful error retry, also flush new events
  instead of skipping the current tick (handler.go:261-271)
- Add test for observeStats retry-then-flush (handler_test.go)

handler/tunnel/id.go

@@ -1,3 +1,7 @@
+// Package tunnel implements the GOST relay tunnel handler for NAT traversal,
+// connecting public entrypoints to internal services behind NAT/firewall.
+//
+// See handler.go for a full architecture overview.
 package tunnel
 
 import (