feat: Add BLE RSSI proximity detector example.

[nedseb]

Target repository

This example should be created in micropython-steami-sample, not in micropython-steami-lib. BLE examples are not tied to a specific driver and belong in the sample repo.

What

Create an example using two STeaMi boards: one broadcasts a BLE beacon, the other scans and displays a proximity gauge on the OLED based on the received RSSI signal strength.

Why

Second step in the BLE + indoor localization path. Introduces the concept of RSSI as a proxy for distance, which is the foundation of BLE indoor positioning.

Suggested approach

1. Beacon board: advertise with a recognizable name (e.g. STeaMi-BEACON)
2. Scanner board: scan for the beacon, read the RSSI value
3. Map RSSI to a proximity level (e.g. -30 dBm = very close, -90 dBm = far)
4. Display a steami_screen.gauge() that fills up as the beacon gets closer
5. Optional: add color coding (GRAY=far, LIGHT=medium, RED=close)

Learning goals

• BLE scanning with aioble
• Understanding RSSI and its relationship to distance
• Signal filtering (RSSI is noisy — consider averaging over N samples)

Hardware

• 2 STeaMi boards (one beacon, one scanner)
• SSD1327 OLED on the scanner board

Depends on

• feat: Add BLE beacon advertising example. #409 (BLE beacon example — reuse the beacon code)

[nedseb]

N'oublie pas de rédiger une fiche pédagogique pour le wiki STeaMi : objectifs pédagogiques, instructions pas-à-pas, résultats attendus et idées d'extension.