Add optional native Linux CPU tuning

[bobmoser76]

Summary:

• add a portable-by-default linux_cpu_optimization Meson option
• support opt-in -march=native and -mtune=native for machine-local builds
• document Arch/CachyOS dependencies and optimized build commands
• clarify that distributable builds must remain portable

Validation:

• fresh Meson debug build configured with linux_cpu_optimization=native
• compiler flag checks passed
• client, dedicated server, and unified game module linked successfully
• git diff --check