feat(ism330dl): Add pedometer (step counter) example.

[nedseb]

What

Create a step counter example that detects footsteps using the ISM330DL accelerometer and displays the count + estimated distance on the OLED.

Why

Practical application of accelerometer data processing. Teaches peak detection, thresholding and debouncing — fundamental signal processing concepts on a tangible use case.

Suggested approach

1. Read `acceleration_g()` continuously from the ISM330DL
2. Compute the magnitude: `mag = sqrt(axax + ayay + az*az)`
3. Detect a step when the magnitude crosses a threshold (e.g. 1.2g) with a minimum interval between steps (debounce ~300ms)
4. Display on the OLED using `steami_screen`:
   • `screen.value(str(steps))` as the main number
   • `screen.subtitle("~{} m".format(steps * STRIDE_M))` for estimated distance
   • `screen.gauge(steps, max_val=TARGET_STEPS)` for a progress arc toward a daily goal
5. `try/finally` with `imu.power_off()`

Learning goals

• Signal magnitude computation
• Peak detection with threshold + debounce
• Real-time counting and display

Hardware

• ISM330DL accelerometer
• SSD1327 OLED

Documentation

Write a pedagogical activity sheet for the STeaMi wiki covering:

• Learning objectives (signal processing, peak detection, debounce)
• Step-by-step instructions
• How to calibrate the threshold and stride length
• Extension ideas (calorie estimation, activity classification walk/run, BLE broadcast to phone)

[nedseb]

Cross-reference with Arduino: steamicc/arduino-steami-lib#75 lists a PedometerSimple example with the same concept. Kaan is assigned on both — the signal-processing logic (threshold + debounce) can be ported directly.