Create Nucleo N657x0-Q Platform

[bremoran]

This change introduces a new bare-metal test platform for the Nucleo N657x0-Q board and integrates it into the project’s Nix devshell and test build system.

Summary

• Adds a dedicated Nix devshell target for nucleo-n657x0-q.
• Introduces a full bare-metal platform under test/baremetal/platform/nucleo-n657x0-q/, including build rules, execution wrapper, argument packing utility, and platform sources.
• Updates HAL and test make components to integrate the new platform.

Changes

• flake.nix: Add .#nucleo-n657x0-q devshell.
• nix/nucleo-n657x0-q/default.nix: Define board devshell and tooling.
• nix/util.nix: Utility update for new platform.
• test/baremetal/platform/nucleo-n657x0-q/README.md: Platform documentation.
• test/baremetal/platform/nucleo-n657x0-q/platform.mk: Makefile integration for the platform.
• test/baremetal/platform/nucleo-n657x0-q/exec_wrapper.py: Execution wrapper for running tests on target.
• test/baremetal/platform/nucleo-n657x0-q/make_argv_bin.py: Helper to pack argv for target.
• test/baremetal/platform/nucleo-n657x0-q/src/*.c, *.h: Platform sources (cmdline, semihosting syscall, helpers).
• test/hal/hal.c, test/hal/pmu_armv8.h: Minor HAL updates for compatibility.
• test/mk/components.mk: Include the new platform components.

Why

• Enables running and validating ML-KEM tests on the Nucleo N657x0-Q board.
• Establishes a consistent devshell and build workflow for this platform.

Usage

• Devshell: nix develop .#nucleo-n657x0-q
• Build tests: make test EXTRA_MAKEFILE=test/baremetal/platform/nucleo-n657x0-q/platform.mk

Notes

• No breaking changes expected to other platforms.
• Documentation included under the platform’s README.md.