[HW][FW] SPI bring-up: ESP32-S3 ↔ ADS1256 + first cell reading

[AldoSeffrin]

DEVELOPMENT_PLAN §1.3 (SPI table) + §2.1 (environment). Second electronics session.

Scope

• PlatformIO project: ESP32-S3, Arduino framework (firmware/ directory)
• ADS1256 on AVDD 5V + SPI wiring: SCLK→18, MOSI→23, MISO→19, CS→5, DRDY→4
• Minimal firmware: read ADS1256 registers over SPI (proof of life)
• Wire 1 load cell (E+, E−, S+, S−) → raw values on serial monitor

Gate: raw values respond to manual load on the cell.
Depends on: #1

Ref: docs/DEVELOPMENT_PLAN.md §1.3 + §2.1.