fix(hop): skip priority short-circuit when multiple nodes share the same priority

When two or more nodes have an identical matcher rule they receive
identical default priority (rule string length). The priority shortcut
assumed the top-priority node was the single authoritative choice and
returned it directly, bypassing the selector (FailFilter, BackupFilter,
strategy) — so weight, round-robin, and hash were silently ignored.

Add a third condition: the top priority must be strictly greater than
the second-highest. When multiple nodes share the same highest priority
the selector applies normally for load balancing.

Add doc comments on NodeMatcherConfig.Priority explaining the three
semantic ranges (0=auto, negative=always-selector, positive=shortcut).

hop/hop_test.go

@@ -522,6 +522,31 @@ func TestSelect_WithSelector(t *testing.T) {
 	}
 }
 
+func TestSelect_EqualPriorityDoesNotShortcut(t *testing.T) {
+	// Two nodes with identical matchers get the same default priority.
+	// The priority shortcut must NOT trigger when multiple nodes share
+	// the highest priority — the selector should still apply.
+	n1 := chain.NewNode("n1", "127.0.0.1:8080",
+		chain.MatcherNodeOption(&testMatcher{match: true}),
+		chain.PriorityNodeOption(50),
+	)
+	n2 := chain.NewNode("n2", "127.0.0.1:9090",
+		chain.MatcherNodeOption(&testMatcher{match: true}),
+		chain.PriorityNodeOption(50),
+	)
+	sel := &testNodeSelector{selectedIdx: 1} // picks n2
+	h := newTestHop(NodeOption(n1, n2), SelectorOption(sel))
+	defer h.Close()
+
+	node := h.Select(context.Background())
+	if node == nil {
+		t.Fatal("expected node, got nil")
+	}
+	if node.Name != "n2" {
+		t.Errorf("expected 'n2' (selected by selector, not shortcut), got %q", node.Name)
+	}
+}
+
 func TestSelect_NilNodesSkipped(t *testing.T) {
 	h := newTestHop()
 	defer h.Close()