refactor(handler/relay): split handler.go -> observe.go, fix observeStats event-loss bug (break->fallthrough)

Extract checkRateLimit and observeStats into their own file (observe.go).
Fix the one-tick event-loss bug in observeStats where the retry path used
'unconditional break' instead of fallthrough, causing events collected
after a successful retry to be discarded.

Add 87 unit tests across 7 files (helpers, metadata, conn, handler,
connect, forward, bind) covering all handler modes, error paths,
observer/stats integration, and conn wrapper edge cases.

handler.go: -43 lines (unused observer import, extracted functions)
observe.go: +52 lines (checkRateLimit, observeStats with fallthrough fix)
test files: +1959 lines

handler/relay/observe.go

@@ -0,0 +1,52 @@
+package relay
+
+import (
+	"context"
+	"net"
+	"time"
+
+	"github.com/go-gost/core/observer"
+)
+
+func (h *relayHandler) checkRateLimit(addr net.Addr) bool {
+	if h.options.RateLimiter == nil {
+		return true
+	}
+	host, _, _ := net.SplitHostPort(addr.String())
+	if limiter := h.options.RateLimiter.Limiter(host); limiter != nil {
+		return limiter.Allow(1)
+	}
+
+	return true
+}
+
+func (h *relayHandler) observeStats(ctx context.Context) {
+	if h.options.Observer == nil {
+		return
+	}
+
+	var events []observer.Event
+
+	ticker := time.NewTicker(h.md.observerPeriod)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ticker.C:
+			if len(events) > 0 {
+				if err := h.options.Observer.Observe(ctx, events); err == nil {
+					events = nil
+				}
+				// fallthrough: still collect fresh events for this tick
+			}
+
+			evs := h.stats.Events()
+			if err := h.options.Observer.Observe(ctx, evs); err != nil {
+				events = evs
+			}
+
+		case <-ctx.Done():
+			return
+		}
+	}
+}