fix(installer): report the requested unsupported platform

[xujiantop-crypto]

When ensureEngine() receives an unsupported platform or architecture override, it selects assets using that override but reports the host platform in the error. For example, requesting linux-riscv64 on Windows x64 reports no CodeGraph engine is published for win32-x64.

Resolve the platform and architecture once, using the existing defaults for omitted values, and use them for both asset selection and the error message. Add regression coverage for three unsupported pairs and each partial override, including rejection before the install directory is created.

Validation:

• Before the fix, the five new error-message assertions fail against main (489ccf1); the existing installer checks pass.
• After the fix, npm test in mcp-package exits successfully on Windows x64 with Node.js 24.10.0. All installer checks pass. The existing wrapper argument suite reports SKIP because its stub engine was not used on this host.
• git diff --check passes.