Guard deep structural equality and comparison recursion (#3539)

jv_equal and jv_cmp overflows the C stack on deeply nested
input. Cap recursion at 10000 with -1 / INT_MIN sentinels;
operators that compose user expressions surface this as
"Equality check too deep" / "Comparison too deep".

Fixes CVE-2026-47770.

tests/jq.test

@@ -2611,3 +2611,25 @@ null
 try (reduce range(10001) as $_ ({}; {a: .}) as $x | $x * $x) catch .
 null
 "Object merge too deep"
+
+# regression test for deep structural equality recursion
+try ((reduce range(10001) as $_ ([]; [.])) as $x | (reduce range(10001) as $_ ([]; [.])) as $y | $x == $y) catch .
+null
+"Equality check too deep"
+
+# regression tests for deep ordering comparisons
+try ((reduce range(10001) as $_ ([]; [.])) as $x | [$x, $x] | sort) catch .
+null
+"Comparison too deep"
+
+try ((reduce range(10001) as $_ ([]; [.])) as $x | [$x, $x] | unique) catch .
+null
+"Comparison too deep"
+
+try ((reduce range(10001) as $_ ({}; {a: .})) as $x | [$x, $x] | sort) catch .
+null
+"Comparison too deep"
+
+try ((reduce range(10001) as $_ ({}; {a: .})) as $x | [$x, $x] | unique) catch .
+null
+"Comparison too deep"