games: Support supervised loadable game processes.

[aviralgarg05]

Note: Please adhere to Contributing Guidelines.

Summary

cgol renders forever and has no exit condition of its own, so once started
there was no way to ask it to stop other than killing the task, which can
land in the middle of a framebuffer write.

This adds a SIGTERM handler that only records the request, with the actual
shutdown taken from the render loop where it is safe to unmap and close.
The Kconfig entries for cgol, match4 and snake also become tristate so the
games can be built as loadable modules rather than only into the image.

Impact

• New feature: The games can be built as loadable modules.
• User adaptation: NO.
• Build: The three Kconfig entries become tristate.
• Hardware: NO.
• Documentation: NO.
• Security: NO.
• Compatibility: Built into the image, behaviour is unchanged apart from
  the game now exiting cleanly on SIGTERM instead of being killed.

Testing

Build host: macOS 26.5, arm64, xtensa-esp-elf-gcc 14.2.0
(esp-14.2.0_20251107).

Target: Xtensa / ESP32-S3, Waveshare ESP32-S3-Touch-LCD-7.

• nxstyle, tools/checkpatch.sh, codespell and git diff --check
• built for the target as builtins and as loadable modules
• on the board, cgol was started, sent SIGTERM, and exited on its own with
  the framebuffer released; ps showed no leftover task afterwards

PR verification Self-Check

• This PR introduces one focused change.
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• I have reviewed and signed every commit.
• This PR adheres to the current contribution and coding guidelines.
• My PR is still work in progress.
• My PR is ready for review and can be safely merged.