Desktop: Build the bundled PHP runtime on Windows

[priethor]

What

Follow-up to #405.

This lets Cortext build and use its own PHP 8.5 runtime on Windows. CI compiles and verifies php.exe, builds the snapshot with it, and then launches Cortext without a system PHP override. The Windows E2E covers startup, persistence, shutdown, and releasing the runtime port.

macOS keeps its existing runtime flow. Packaging and signing remain separate follow-ups.

Why

#405 made the snapshot builder work on Windows, but the desktop app still could not start there without a bundled PHP runtime. Shipping a known PHP version with Cortext means Windows users will not need to install or configure PHP themselves.

Testing Instructions

1. On Windows x64 with Visual Studio 2022 and Git for Windows installed, build the bundled PHP runtime from apps/desktop. Confirm that it creates runtime/bin/php.exe and reports PHP 8.5.
2. Build the snapshot with that runtime, then launch Cortext without setting CORTEXT_PHP_BIN. Confirm that the app opens normally.
3. Create or edit some content, quit Cortext, and open it again. Confirm that the change is still there.
4. Quit Cortext and confirm that its php.exe process exits and the runtime port is no longer in use.