fix: make cutover restart argv upgrade-safe and tests platform-honest

- cutover-orchestrator: restart via 'node --import tsx' (matching install.sh)
  instead of a hardcoded .pnpm/tsx@4.22.4 loader path that breaks on tsx bumps
- production-gateway: use process.execPath for the legacy-start fixture;
  POSIX-only 0600 mode assertion now win32-guarded
- cli-runtime + installer suites: skip where they require POSIX-only
  process-group SIGTERM, lsof, or /bin/sh
- fixture-safety: fall back to config.example.json when config.json is absent
  on a fresh clone

apps/api/src/cutover-orchestrator.ts

@@ -23,7 +23,7 @@ const STATE_FILE = 'cutover-state.json';
 const GATEWAY_ARGV = [
   process.execPath,
   '--import',
-  resolve(process.cwd(), 'node_modules/.pnpm/tsx@4.22.4/node_modules/tsx/dist/loader.mjs'),
+  'tsx',
   resolve(process.cwd(), 'apps/api/src/production-gateway-cli.ts'),
 ] as const;
 const DIGEST = /^[a-f0-9]{64}$/u;