G3P-22238: Revert to opt-in privilege model in osThreadNew

[nickfritz33]

Summary

• Restore the original opt-in check (osThreadPrivileged must be explicitly set) instead of the opt-out inversion. With CREATE_PRIVILEGED_THREAD_ATTRIBUTES setting osThreadPrivileged on all service threads and CREATE_THREAD_ATTRIBUTES leaving it unset for unprivileged tasks, opt-in is the correct and cleaner default.

Context

This work supports G3P-22238, which creates an unprivileged FreeRTOS task on STM32H573 (Cortex-M33 with TrustZone). The CMSIS-RTOS2 wrapper previously had no mechanism to create unprivileged tasks via the `osThreadNew` API — all threads were created privileged regardless of the `osThreadUnprivileged` attribute flag.

Test plan

• Branch firmware (STM32H573, `configENABLE_MPU=1`) builds clean
• Counter task runs as unprivileged on Branch hardware without MPU faults
• System regression tests completed without issue

🤖 Generated with Claude Code

[nickfritz33]

Here's an output from GDB showing the task continuously executing the unprivileged thread.
G3P-22238 UPL task running (GDB bp confirmed).rtf

[austinladshaw]

good catch