perf(cli): skip native-binary build tests by default

The build-exe and build-exe-cross suites invoke `bun build` in beforeAll
hooks and pegged CPU for ~6 min on every test run (one cached-on-miss
build plus three unconditional cross-target builds). Move them to a
dedicated vitest project, skip them unless CI=1 or FUSION_TEST_BUILD_EXE=1
is set, and add cache guards so CI re-runs skip rebuilds when the target
binary already exists. With build-exe out of the default cli run, the
main config can enable fileParallelism.

Run on demand via `pnpm --filter @runfusion/fusion test:build-exe`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/cli/package.json

@@ -42,7 +42,8 @@
     "build:exe": "bun run build.ts",
     "build:exe:all": "bun run build.ts --all",
     "typecheck": "tsc --noEmit",
-    "test": "vitest run --silent=passed-only --reporter=dot"
+    "test": "vitest run --silent=passed-only --reporter=dot",
+    "test:build-exe": "FUSION_TEST_BUILD_EXE=1 vitest run --config vitest.build-exe.config.ts --silent=passed-only --reporter=dot"
   },
   "dependencies": {
     "@fusion/pi-claude-cli": "workspace:*",