Add regression test: re-binding a singleton to a different path must keep it a singleton (#3516) #3527

Summary

#3520 fixed the Injector.map() lifecycle regression (#3516), but no spec pins the exact failure mode. The two existing re-bind tests both re-apply .asSingleton(), so neither would catch the flag being dropped:

• "invalidates a cached singleton when the alias is re-bound to a different component path" → re-maps then calls .asSingleton() again
• "keeps the cached singleton when the alias is re-registered with the same path (dev-reload pattern)" → same path

The regression was specifically: re-pointing an existing singleton to a different component without re-flagging silently degraded it to transient (map() deleted singletonFlags on the path re-map branch). A test that omits the second .asSingleton() is what pins the fix.

Suggested regression test

Add to vendor/wheels/tests/specs/di/InjectorSpec.cfc (Core API describe):

it("preserves the singleton lifecycle when re-bound to a different path WITHOUT re-flagging (##3516)", () => {
    di.map("repointedSingleton").to("wheels.tests._assets.di.SimpleService").asSingleton();
    di.getInstance("repointedSingleton");
    // Re-point to a DIFFERENT component without re-applying .asSingleton().
    // The lifecycle must survive the re-map (the test-double DI-swap pattern).
    di.map("repointedSingleton").to("wheels.tests._assets.di.OptionalDependentService");

    expect(di.isSingleton("repointedSingleton")).toBeTrue();
    var first = di.getInstance("repointedSingleton");
    var second = di.getInstance("repointedSingleton");
    expect(first).toBe(second);
    expect(structKeyExists(first, "hasDependency")).toBeTrue();
});

Fails on the pre-#3520 code (isSingleton() returns false) and passes after #3520.