feat(rpn): handle @-prefixed standalone metrics (@GB, @Mbps)

Token @GB pushes Number(1, mode, GB) onto the stack.
Uses FindWithAliases for metric lookup (exact, alias, case-insensitive).
Returns error for unknown metrics.
No conflict with : symbols (@ is distinct from : prefix).

Includes unit and integration tests.

2 files changed, 77 insertions, 0 deletions

diff --git a/internal/rpn/metric_test.go b/internal/rpn/metric_test.go
index 88b8c3b..9eb1cf7 100644
--- a/internal/rpn/metric_test.go
+++ b/internal/rpn/metric_test.go
@@ -780,3 +780,66 @@ func TestParseNumberWithMetricExactMatch(t *testing.T) {
 		t.Error("parseNumberWithMetric(100Bps) should fail (B = bytes)")
 	}
 }
+
+func TestAtPrefixMetricParsing(t *testing.T) {
+	reg := GetMetricRegistry()
+
+	tests := []struct {
+		token      string
+		wantMetric string
+		wantOK     bool
+	}{
+		{"@GB", "GB", true},
+		{"@Mbps", "Mbps", true},
+		{"@hr", "hr", true},
+		{"@sec", "s", true},  // alias
+		{"@foot", "ft", true}, // alias
+		{"@nope", "", false},
+		{"@", "", false},
+	}
+
+	for _, tt := range tests {
+		// Simulate the @ prefix logic from rpn_parse.go
+		if len(tt.token) <= 1 || tt.token[0] != '@' {
+			continue
+		}
+		metricName := tt.token[1:]
+		metric, ok := reg.FindWithAliases(metricName)
+		if ok != tt.wantOK {
+			t.Errorf("@%s ok = %v, want %v", metricName, ok, tt.wantOK)
+			continue
+		}
+		if ok && tt.wantMetric != "" {
+			expected, _ := reg.Find(tt.wantMetric)
+			if metric != expected {
+				t.Errorf("@%s = %q, want %q", metricName, metric.Name, tt.wantMetric)
+			}
+		}
+	}
+}
+
+func TestAtPrefixIntegration(t *testing.T) {
+	// Test that @GB parses correctly through the full RPN pipeline
+	vars := NewVariables()
+	rpn := NewRPN(vars)
+
+	// Parse a standalone @ metric
+	result, err := rpn.ParseAndEvaluate("@GB")
+	if err != nil {
+		t.Fatalf("ParseAndEvaluate(@GB) failed: %v", err)
+	}
+	// Should push 1 with GB metric; display is "1"
+	if result != "1" {
+		t.Errorf("@GB result = %q, want %q", result, "1")
+	}
+
+	// The stack should have a number with the GB metric
+	stack := rpn.GetCurrentStack()
+	if len(stack) != 1 {
+		t.Fatalf("expected 1 item on stack, got %d", len(stack))
+	}
+	m := stack[0].Metric()
+	if m == nil || m.Name != "GB" {
+		t.Errorf("stack[0].Metric() = %v, want GB", m)
+	}
+}
diff --git a/internal/rpn/rpn_parse.go b/internal/rpn/rpn_parse.go
index 40ff432..f115090 100644
--- a/internal/rpn/rpn_parse.go
+++ b/internal/rpn/rpn_parse.go
@@ -335,6 +335,20 @@ func (r *RPN) evaluate(input string, tokens []string) (string, error) {
 			continue
 		}
 
+		// Check for @ prefix: standalone metric (e.g., @GB, @Mbps)
+		// Pushes a Number with value 1 and the looked-up metric
+		if len(token) > 1 && token[0] == '@' {
+			metricName := token[1:]
+			if metric, ok := GetMetricRegistry().FindWithAliases(metricName); ok {
+				if stack.Len() >= r.maxStack {
+					return "", fmt.Errorf("stack overflow")
+				}
+				stack.Push(NewNumber(1, r.mode, metric))
+				continue
+			}
+			return "", fmt.Errorf("unknown metric %q in %q", metricName, token)
+		}
+
 		// Check if this is a variable name for assignment (:= or =:)
 		// For := (right assignment): name value := - first token is always a variable name
 		// For =: (left assignment): value name =: - token before =: is a variable name