feat: add flat include interface via forwarding headers

[MatejGomboc-Claude-MCP]

Summary

This PR introduces a flat include interface for the library, allowing users to include headers using a clean, unified syntax:

#include "armcortex/mpu.hpp"
#include "armcortex/nvic.hpp"
#include "armcortex/barriers.hpp"
#include "armcortex/systick.hpp"

Instead of the previous architecture-specific paths like m0plus/mpu.hpp.

How it works

The include/armcortex/ directory contains thin forwarding headers that use preprocessor conditionals to include the correct architecture-specific implementation based on the ARM_CORTEX_Mx compile definition (which is already set by CMake).

For example, include/armcortex/mpu.hpp:

#pragma once

#if defined(ARM_CORTEX_M0PLUS)
#include "../../m0plus/mpu.hpp"
#elif defined(ARM_CORTEX_M3)
#include "../../m3/mpu.hpp"
#elif defined(ARM_CORTEX_M0) || defined(ARM_CORTEX_M1)
#error "MPU is not available on Cortex-M0 and Cortex-M1"
#else
#error "No ARM Cortex-M architecture defined..."
#endif

Benefits

• Clean user experience: Simple, flat includes for end users
• Clear error messages: Helpful compile-time errors if a feature isn't available for the target architecture
• No restructuring: Original source files remain in their organized directories (m0/, m0plus/, m3/, intrinsics/)
• No code generation: Forwarding headers are hand-written and version controlled
• IDE friendly: All headers are listed in target_sources for visibility

Available headers

Header	Description	Architectures
armcortex/nvic.hpp	Nested Vectored Interrupt Controller	All
armcortex/scb.hpp	System Control Block	All
armcortex/systick.hpp	SysTick Timer	All
armcortex/exceptions.hpp	Exception numbers	All
armcortex/special_regs.hpp	Special registers	All
armcortex/mpu.hpp	Memory Protection Unit	M0+, M3+
armcortex/scnscb.hpp	System Control (not in SCB)	M1 only
armcortex/barriers.hpp	Memory barriers	All
armcortex/byte_reversing.hpp	Byte swap intrinsics	All
armcortex/debugging.hpp	Debug intrinsics	All
armcortex/hints.hpp	Compiler hints	All
armcortex/power_management.hpp	Power management	All
armcortex/bit_operations.hpp	Bit manipulation	M3+ (ARMv7-M)
armcortex/exclusive_access.hpp	Exclusive load/store	M3+ (ARMv7-M)
armcortex/saturation.hpp	Saturating arithmetic	M3+ (ARMv7-M)
armcortex/bit_utils.hpp	Bit utilities	All
armcortex/intrinsics.hpp	All intrinsics (convenience)	All

Testing

The CI should verify that the new include paths work correctly for all architecture configurations.