On real hardware, ESP_TIMER_ISR callbacks run in a hardware interrupt
that preempts any FreeRTOS task. On the Linux simulator, there are no
real ISRs — the alarm is detected by a native pthread but was only
forwarded to the FreeRTOS timer_task via xTaskNotifyGive(). This meant
ISR-dispatch callbacks could be starved by higher-priority FreeRTOS
tasks, breaking components like the task watchdog that rely on
ISR-dispatch timers to detect scheduling starvation.
Move ISR-dispatch timer processing into the alarm pthread itself,
mirroring the hardware ISR path. The FreeRTOS timer_task is only
notified when no ISR-dispatch timer consumed the alarm. This is safe
because the Linux FreeRTOS port already handles vPortEnterCritical()
calls from non-FreeRTOS threads (bumps nesting counter without
blocking on scheduled-task checks).
