feat(core-typescript): split vitest base into node + jsdom flavors

Adds vitest.base.node and vitest.base.jsdom with safety defaults
(clearMocks, restoreMocks, mockReset, unstubGlobals, sequence.shuffle)
and coverage thresholds (80/75/80/80). Migrates all feature configs
to the new base. Existing baseVitestConfig kept as backwards-compat
re-export of nodeVitestConfig.

Spec: §6.2

packages/core-shared/package.json

@@ -23,6 +23,7 @@
   },
   "devDependencies": {
     "@repo/core-eslint": "workspace:*",
+    "@repo/core-testing": "workspace:*",
     "@repo/core-typescript": "workspace:*",
     "@types/node": "^22.0.0",
     "vitest": "^3.1.0"

packages/core-shared/vitest.config.ts

@@ -1,3 +1,9 @@
-import { baseVitestConfig } from "@repo/core-typescript/vitest.base";
+import path from "node:path";
+import { mergeConfig } from "vitest/config";
+import { nodeVitestConfig } from "@repo/core-typescript/vitest.base.node";
 
-export default baseVitestConfig;
+export default mergeConfig(nodeVitestConfig, {
+  resolve: {
+    alias: { "@": path.resolve(__dirname, "./src") },
+  },
+});