fix(core): 同一 loader 版本 id 撞两条父链时具名报错，不覆盖盘上清单（旧实例会静默换父链）

crates/core/tests/liteloader/guards.rs

@@ -155,3 +155,58 @@ fn cancel_leaves_no_manifest() {
         unlucky_core::game::GameError::MissingParent { .. }
     ));
 }
+
+/// 同一个 loader 版本 id 被两条父链请求：**具名报错**，且盘上那份清单一个字节都不动。
+///
+/// 为什么必须有这道门：`<原版>-liteloader-<ver>` 对「叠原版」与「叠 Forge」是同一个目录名，
+/// 直接覆盖会让已经登记、指向这个版本 id 的旧实例**静默换父链**（启动行为跟着变）。
+#[test]
+fn parent_collision_is_refused_instead_of_overwriting() {
+    let temp = TempDir::new("parent-collision");
+    let layout = GameLayout::new(temp.path());
+    put_version(&layout, "1.12.2", VANILLA_1122);
+    put_version(
+        &layout,
+        "1.12.2-forge-14.23.5.2860",
+        crate::support::FORGE_1122,
+    );
+
+    let vanilla_release = liteloader_release("1.12.2");
+    let existing = serde_json::json!({
+        "id": vanilla_release.version_id,
+        "inheritsFrom": "1.12.2",
+        "mainClass": "net.minecraft.launchwrapper.Launch",
+        "arguments": {"game": ["--tweakClass", "com.mumfrey.liteloader.launch.LiteLoaderTweaker"]},
+        "libraries": []
+    });
+    put_version(
+        &layout,
+        &vanilla_release.version_id,
+        &serde_json::to_string_pretty(&existing).expect("序列化"),
+    );
+    let before = std::fs::read_to_string(layout.version_json(&vanilla_release.version_id))
+        .expect("读旧清单");
+
+    let forge_release = liteloader_release("1.12.2-forge-14.23.5.2860");
+    assert_eq!(
+        forge_release.version_id, vanilla_release.version_id,
+        "两条父链撞同一个 id"
+    );
+    let error = LoaderInstallPlan::build(LoaderPlanInput {
+        layout: &layout,
+        release: &forge_release,
+        environment: &Environment::host(),
+        parent_installed: true,
+    })
+    .expect_err("撞父链必须报错");
+    assert_eq!(error.kind(), LoaderErrorKind::Unsupported);
+    assert!(
+        error.message().contains("1.12.2-forge-14.23.5.2860") && error.message().contains("1.12.2"),
+        "要说清两条父链各是什么：{}",
+        error.message()
+    );
+
+    let after = std::fs::read_to_string(layout.version_json(&vanilla_release.version_id))
+        .expect("读旧清单");
+    assert_eq!(before, after, "报错之后盘上那份清单不许被改动");
+}