feat: add standalone build system and roundtrip tests (Issue #17)

- CMakeLists.txt: builds turboquant as static library
- TURBOQUANT_BUILD_TESTS option enables ctest roundtrip tests
- tests/roundtrip_test.cpp: validates zero-vector roundtrip and
  gaussian cosine similarity (>=0.99)
- Makefile wrapper for convenience (build/test/clean targets)
- Addresses contributor feedback on spec-to-code gap and CI from #17

tests/test_standalone_build.py

@@ -0,0 +1,64 @@
+import pathlib
+import shutil
+import subprocess
+import tempfile
+import unittest
+
+
+REPO_ROOT = pathlib.Path(__file__).resolve().parents[1]
+
+
+class StandaloneBuildTest(unittest.TestCase):
+    def test_cmake_builds_and_runs_ctest(self) -> None:
+        build_dir = pathlib.Path(tempfile.mkdtemp(prefix="turboquant-cmake-"))
+        self.addCleanup(lambda: shutil.rmtree(build_dir, ignore_errors=True))
+
+        configure = subprocess.run(
+            [
+                "cmake",
+                "-S",
+                str(REPO_ROOT),
+                "-B",
+                str(build_dir),
+                "-DTURBOQUANT_BUILD_TESTS=ON",
+            ],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+        if configure.returncode != 0:
+            self.fail(
+                "cmake configure failed\n"
+                f"stdout:\n{configure.stdout}\n"
+                f"stderr:\n{configure.stderr}"
+            )
+
+        build = subprocess.run(
+            ["cmake", "--build", str(build_dir)],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+        if build.returncode != 0:
+            self.fail(
+                "cmake build failed\n"
+                f"stdout:\n{build.stdout}\n"
+                f"stderr:\n{build.stderr}"
+            )
+
+        ctest = subprocess.run(
+            ["ctest", "--test-dir", str(build_dir), "--output-on-failure"],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+        if ctest.returncode != 0:
+            self.fail(
+                "ctest failed\n"
+                f"stdout:\n{ctest.stdout}\n"
+                f"stderr:\n{ctest.stderr}"
+            )
+
+
+if __name__ == "__main__":
+    unittest.main()