Feat/ble trilateration.

[MatteoCnda1]

Summary

Add a BLE trilateration indoor positioning example using aioble and
steami_screen. Closes #413

Changes

• Added BLE/Trilateration_positioning/main.py: single-file example running on
  4 STeaMi boards (3 beacons + 1 mobile)
• Mode selection via D-PAD menu:
  • BEACON mode: select beacon ID (M1/M2/M3), advertise continuously,
    LEFT to return to menu
  • MOBILE mode: scan all 3 beacons, trilaterate position, display
    live dot on OLED map
• Per-beacon path loss model calibrated on site:
  • RSSI measured at 1m and 2m with nRF Connect
  • n computed per beacon: M1=3.99, M2=4.32, M3=2.66
• Multi-layer filtering pipeline:
  • Moving average on RSSI (8 samples)
  • Distance clamped to 430cm to avoid aberrant values
  • Trilateration result rejected if too far from centroid
  • Exponential smoothing on position (alpha=0.15)
  • Minimum movement threshold (15cm) to suppress jitter
• Beacon coordinates measured on site (triangle formation):
  • M1=(0,0), M2=(420,0), M3=(330,278) in cm

Checklist

• Tested on hardware (STeaMi board)
• Position accurate near beacons, stable at triangle center
• Examples added (BLE/Trilateration_positioning/main.py)
• Commit messages follow <scope>: <Description.> format