feat(firmware): implement PM_STATE_SUSPEND_TO_IDLE for light sleep

[jknoxdev]

hw_enter_light_sleep() in firmware/src/main.c:323 contains a
TODO for PM_STATE_SUSPEND_TO_IDLE — the CPU never actually enters
a low-power state during the light sleep window.

Implementation:
Wire pm_state_force(0, PM_STATE_SUSPEND_TO_IDLE) after sensor IRQs
are confirmed active. CPU should wake on sensor interrupt or inactivity
timeout.

Acceptance:
PPK2 shows reduced current draw during light sleep window vs ARMED.

label: enhancement