Add edge case tests for expression evaluator (code review)

Test cases for:
- Unary minus in expression: 3*-2 -> -6
- Parenthesized negative: (-5) -> -5
- Unary minus after power: 2^-1 -> 0 (truncated)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/formula/range_test.go

@@ -91,6 +91,21 @@ func TestEvaluateExpr(t *testing.T) {
 			expr: "1-10",
 			want: -9,
 		},
+		{
+			name: "unary minus in expression",
+			expr: "3*-2",
+			want: -6,
+		},
+		{
+			name: "parenthesized negative",
+			expr: "(-5)",
+			want: -5,
+		},
+		{
+			name: "unary minus after power",
+			expr: "2^-1",
+			want: 0, // 0.5 truncated to int
+		},
 		{
 			name: "division by zero",
 			expr: "10/0",