schemagen: reject scalar root values when root schema is object/array

The generated RootBuilder crashed (undefined behavior on std::vector::back())
when a JSON document's root value was a scalar or null while the schema
declared a non-nullable object or array root. The stack starts empty for
object/array roots, but cbStringData, cbNumberData, and cbBool called
stack_.back() without checking for an empty stack.

Add an empty-stack guard to the three scalar callbacks so they reject
instead of crashing. cbNull already handles the empty-stack case.

Regression tests added for:
- non-nullable object root rejecting null, boolean, number, and string roots
- nullable object root rejecting scalar roots
- nullable array root rejecting scalar roots

Closes #16

contrib/schemagen/test_gen.cpp

@@ -184,6 +184,10 @@ int main() {
   expectReject(R"({"name":"x","age":1,"address":{"zip":5}})",
                "missing required nested 'city'");
   expectReject(R"([1,2,3])", "array where object expected (root)");
+  expectReject("null", "null where object expected (root)");
+  expectReject("true", "boolean where object expected (root)");
+  expectReject("123", "number where object expected (root)");
+  expectReject(R"("hi")", "string where object expected (root)");
   expectReject(R"({"name":"x","age":1,)", "truncated / invalid json");
 
   if (failures == 0) {