* test(clients): verify native Gemini CLI integration on Windows
The Gemini CLI client lane asserted POSIX. On Windows it also needs the npm install's JavaScript entry point, because only a gemini.cmd shim reaches PATH, and two setup-plan checks compared paths with a hard-coded slash. Remove the assertion, run a selected .js, .mjs or .cjs entry point under the verifier's Node, build those prefixes with path.sep, name the entry point in the Windows preflight error, and record 0.63.0 as verified on native Windows in docs/testing.md.
Fixes#1935
* test(clients): resolve the selected Gemini CLI entry point once
The preflight runs from the caller's directory and the client run from the disposable workspace, so a relative REA_VERIFY_GEMINI_COMMAND passed the preflight and then failed with Cannot find module. Resolve a selected JavaScript entry point to an absolute path before both launches.