Add system time interface for reading RTOS tick count

[Copilot]

DMOSI lacked any interface to read the current RTOS system clock (SysTick), making it impossible for consumers to query elapsed time or implement tick-based logic through the OSI abstraction layer.

Changes

• include/dmosi.h – New DMOSI_SYSTIME_API section exposing dmosi_get_tick_count(), returning the uint32_t RTOS tick count elapsed since scheduler start
• src/dmosi.c – Weak default implementation returning 0; overridden by the RTOS port (e.g. xTaskGetTickCount() on FreeRTOS, osKernelGetTickCount() on CMSIS-RTOS2)

Usage

uint32_t t0 = dmosi_get_tick_count();
// ... do work ...
uint32_t elapsed_ticks = dmosi_get_tick_count() - t0;

Original prompt

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This section details on the original issue you should resolve

<issue_title>Dodać interfejs do pobierania system time</issue_title>
<issue_description>Potrzebny jest interfejs, który nam zwróci aktualny stan systemowego zegara (użytego przez RTOS - SysTick)</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Dodać interfejs do pobierania system time #23

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