schemagen: reserve generated Root/Skip/RootScalar and avoid Kind/ArrN collisions

Make the type-name allocator aware of the identifiers the generator emits
itself (`Root` alias, `Skip`/`RootScalar` Kind enumerators) so user `$defs`
names can no longer collide with them.  Array-kind names (`Arr0`, `Arr1`, ...)
are now allocated only after checking for object/enum names, preventing
duplicate `Kind` enumerators when a schema defines e.g. `Arr0`.

Add Python regression tests that also syntax-check the generated headers
with a C++ compiler.

Closes #21

contrib/schemagen/test_schemagen.py

@@ -3,6 +3,8 @@
 
 import json
 import os
+import re
+import shutil
 import subprocess
 import sys
 import tempfile
@@ -87,5 +89,110 @@ class SchemagenKeywordTest(unittest.TestCase):
             self.assertNotIn(f"std::optional<std::string> {kw};", stdout)
 
 
+class SchemagenCollisionTest(unittest.TestCase):
+    """Regression tests for issue #21: generated Root alias / Kind enum collisions."""
+
+    def setUp(self):
+        self.repo_root = os.path.dirname(
+            os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+        )
+        self.include_dir = os.path.join(self.repo_root, "include")
+        self.compiler = shutil.which("c++")
+
+    def generate_and_compile(self, schema):
+        """Run schemagen on schema and syntax-check the resulting header."""
+        with tempfile.TemporaryDirectory() as tmpdir:
+            schema_path = os.path.join(tmpdir, "schema.json")
+            with open(schema_path, "w") as fp:
+                json.dump(schema, fp)
+            header_path = os.path.join(tmpdir, "gen.h")
+            cmd = [
+                sys.executable,
+                SCRIPT,
+                schema_path,
+                "-o",
+                header_path,
+                "--namespace",
+                "test_schema",
+            ]
+            result = subprocess.run(cmd, capture_output=True, text=True, check=False)
+            self.assertEqual(result.returncode, 0, msg=result.stderr)
+
+            if self.compiler:
+                cpp_path = os.path.join(tmpdir, "test.cpp")
+                with open(cpp_path, "w") as fp:
+                    fp.write(
+                        '#include "gen.h"\n'
+                        "int main() {\n"
+                        "  test_schema::RootBuilder b;\n"
+                        "  test_schema::Root r = b.take();\n"
+                        "  (void)r;\n"
+                        "}\n"
+                    )
+                comp = subprocess.run(
+                    [
+                        self.compiler,
+                        "-std=c++20",
+                        "-fsyntax-only",
+                        "-I",
+                        self.include_dir,
+                        "-I",
+                        tmpdir,
+                        cpp_path,
+                    ],
+                    capture_output=True,
+                    text=True,
+                    check=False,
+                )
+                self.assertEqual(comp.returncode, 0, msg=comp.stderr)
+
+            with open(header_path) as fp:
+                return fp.read()
+
+    def test_root_alias_does_not_collide_with_user_type(self):
+        schema = {
+            "type": "array",
+            "items": {"$ref": "#/$defs/Root"},
+            "$defs": {"Root": {"enum": ["a", "b"]}},
+        }
+        out = self.generate_and_compile(schema)
+        self.assertIn("enum class Root1 : int { a, b };", out)
+        self.assertIn("using Root = std::vector<Root1>;", out)
+        self.assertNotIn("using Root = std::vector<Root>;", out)
+
+    def test_kind_enum_does_not_duplicate_arr0(self):
+        schema = {
+            "type": "object",
+            "properties": {
+                "arr": {"type": "array", "items": {"type": "string"}},
+                "obj": {"$ref": "#/$defs/Arr0"},
+            },
+            "$defs": {"Arr0": {"type": "object", "properties": {}}},
+        }
+        out = self.generate_and_compile(schema)
+        self.assertIn("struct Arr0", out)
+        m = re.search(r"enum class Kind : uint8_t \{([^}]+)\}", out)
+        self.assertIsNotNone(m)
+        enumerators = [e.strip() for e in m.group(1).split(",")]
+        self.assertIn("Arr0", enumerators)
+        self.assertIn("Arr1", enumerators)
+        self.assertEqual(len(enumerators), len(set(enumerators)))
+
+    def test_skip_is_reserved(self):
+        schema = {
+            "type": "array",
+            "items": {"$ref": "#/$defs/Skip"},
+            "$defs": {"Skip": {"type": "object", "properties": {}}},
+        }
+        out = self.generate_and_compile(schema)
+        self.assertIn("struct Skip1", out)
+        m = re.search(r"enum class Kind : uint8_t \{([^}]+)\}", out)
+        self.assertIsNotNone(m)
+        enumerators = [e.strip() for e in m.group(1).split(",")]
+        self.assertIn("Skip", enumerators)
+        self.assertIn("Skip1", enumerators)
+        self.assertEqual(len(enumerators), len(set(enumerators)))
+
+
 if __name__ == "__main__":
     unittest.main()