Adds top-level CMake build (T-04): static JTest lib, example, ctest smoke

The project was Makefile-only and non-portable. Adds a top-level
CMakeLists.txt so the framework builds with a standard build system:

* add_library(JTest STATIC ...) over the 13 src/JTest/*.cpp translation
  units. No find_package/FetchContent is needed: a full #include scan of
  include/, src/, examples/ shows the framework depends only on the C++
  standard library plus its own <JTest/...> headers.
* target_include_directories(JTest PUBLIC include) so consumers that link
  JTest (the example, tests) inherit the include path transitively.
* target_compile_features(JTest PUBLIC cxx_std_17) propagates the C++17
  requirement (sources use std::optional).
* Portable (no -arch) build: the Makefile's non-portable -arch flag is not
  carried over (that is T-20's scope).
* enable_testing() + add_test() running tests/smoke.cpp, a genuine self-check
  that exercises the public API and exits non-zero on any failed assertion, so
  ctest is a real pass/fail gate. The demo example remains a build/run target
  but is intentionally excluded from the CTest gate (it contains deliberate
  failing/skipping cases to showcase reporting and always exits 0).

Verified end-to-end (CMake 4.4.3, AppleClang 21, out-of-source build):
configure OK, builds libJTest.a + example + test_smoke, ctest passes
(smoke: Passed, 100%), example demo runs.

tests/smoke.cpp

@@ -0,0 +1,53 @@
+// Smoke test for the JTest build system.
+//
+// This is a *genuine* self-check (not a demo). It exercises the public API the
+// same way a user program would (describe/it/expect/toEqual/execute) and drives
+// `ctest` so a broken or misconfigured build goes red. Unlike
+// examples/example.cpp (which intentionally contains failing/skipping cases to
+// showcase JTest's reporting and always returns 0), this test returns a
+// non-zero exit code when an assertion fails, so CTest produces a real
+// pass/fail.
+//
+// It links JTest and compiles without any external dependency, proving that the
+// library archives, the include directory is exported transitively, and C++17
+// is in effect.
+#include <JTest/JTest.h>
+
+#include <string>
+
+namespace {
+using JTest::describe;
+using JTest::execute;
+using JTest::expect;
+using JTest::it;
+using JTest::TestBundle;
+using JTest::TestResults;
+using std::string;
+
+int add(int a, int b) {
+    return a + b;
+}
+}   // namespace
+
+int main() {
+    TestResults results = execute(
+        describe("JTest smoke", []() {
+            return TestBundle(
+                {
+                    it("executes a passing arithmetic assertion", []() {
+                        expect(add(2, 2)).toEqual(4);
+                     }),
+                    it("executes a passing string assertion", []() {
+                        expect(string("JTest")).toEqual(string("JTest"));
+                     }),
+                    it("executes a second passing arithmetic assertion", []() {
+                        expect(add(10, 20)).toEqual(30);
+                     }),
+                 },
+                JTest::DescribeOptions());
+         }));
+
+    // Non-zero exit when any test failed or errored, so CTest surfaces a real
+    // failure instead of silently passing.
+    return results.failed == 0 ? 0 : 1;
+}